Roa was originally a priest of a Church who quickly rose to the top by utilizing all his father's inheritance. He created the "burial class", which served as the base of today's Burial Agency, along with Nabareck. With nothing left of his resources, he decided to leave the Church, giving the Agency to Nabareck afterwards. After betraying the Church to the Mage's Association, he was left uncredited, and Riesbyfe Strydeberg was surprised to hear about his involvement with it. He wished to realize an immortality different from that of Dead Apostles, but, in an attempt to overcome his limits as a mere human, he became a vampire in order to continue his research. When he considered a way to become a Dead Apostle, his heart snatched the moment he set his eyes on Arcueid Brunestud, and his plan went out of control thereafter without knowing it himself. He genuinely fell in love with her without noticing himself, but instead started hating her after it caused him to lose his innocence and turn away from his research. Upon her becoming weak to her vampiric impulse for the first time, he tricked her into drinking his blood, and became a powerful Dead Apostle. Taking the forces of a Dead Apostle Ancestor that had been sealed by the Church, he quickly rose in power among the twenty-seven. As he built up his power, he earned himself the scorn of the other Dead Apostles Ancestors, and eventually Altrouge Brunestud was left in charge of punishing this newcomer. He proved more powerful in battle, defeating her without killing her, and afterwards, the rest of the Ancestors decided to ignore his existence, and he was eventually destroyed by Arcueid who had joined forces with the Church. Over the next seven hundred years, he reincarnated fifteen times, and was endlessly killed by Arcueid each time.
Roa's seventeenth reincarnation, Elesia, who was later known as Ciel, was born in 1976 to a baker's family in a remote village in France after his sixteenth incarnation died. The sixteenth was destroyed by Arcueid before he had a chance to find a family bloodline with both wealth and power. He was only able to fulfill the requirement that his next incarnation would have physical potential, so he was born to a normal family without any social standing. In exchange, her body instead had high magical potential, which is the closest to power to his original body in all of his incarnations. Even though he would have to take a long period of time to take over the town without status, he was fine with waiting until later to obtain social standing. He caused various atrocities to the citizens as he used his new powers to slowly take over the town. After the town fell completely under his control, Arcueid inadverdently hunted him down and managed to find him after two days of searching. He was killed by her afterwards, after which Elesia's body was taken to the Church. She eventually revived with her original personality without any of Roa's influence, while Roa moved on to his next reincarnation. He managed to plan his eighteenth incarnation, and chose the Tohno bloodline with a proper amount of wealth and power. Choosing the Far East for his seventeenth revival was on a whim which was possibly just him wishing for a change of scenery, comparable to where his previous incarnations were born.
SHIKI Tohno, who was different from the main protagonist of Tsukihime, Shiki Tohno, is Roa's eighteenth incarnation. He is the true son of the family, who was close to his sister, Akiha, and good friends with his adopted brother from the Nanaya clan. The three would often play together, especially enjoying playing a game where they would mark territory inside the mansion by carving their names on walls, food, and pillars. He eventually fell to his Inversion Impulse, causing him to attack his own sister Akiha. She was protected by Shiki, who suffered from a nearly fatal wound after which he had a part of his "life" stolen by SHIKI. As with the traditions of the Tohno household, SHIKI was to be killed by his father, Makihisa Tohno, but managed to revive using Shiki's "life." He regained some of his sanity, leaving Makihisa unsure about killing him. While he was officially declared deceased to the rest of the family and replaced by Shiki, he was actually kept locked in the Tohno Mansion's cellar. Makihisa planned to bring him back into the family after he was healed, but he remained locked up for around eight years. After Makihisa died, Kohaku started taking good care of SHIKI, planning to use him in her revenge against the family. He is eventually released before the start of Tsukihime.
It seems that Roa was popular as a virtuous man when he was a priest up until his betrayal against the Church. He seems to have a great dislike for the Church itself, commenting that the already ruined Church has degraded even further in eight hundred years, but also says the existence of a woman like Riesbyfe makes it so it's not quite as bad. He had some form of a relationship with Nabareck, as, while she did not want to leave the Church nor live with Roa, she was willing to pass on the information to her child to ignore a newcomer Dead Apostle Ancestor within the next one hundred years.
Roa is an acquaintance of Nrvnqsr Chaos, and they are close enough to one another to call each other friends. Nrvnqsr believes that he understands Roa more than the other Ancestors, having talked extensively about their own meanings of eternity and the fact that Roa taught him the "Soil of Genesis" with the idea of having him use it to capture Arcueid. While they are both heretics by the standards of vampires, Nrvnqsr states that each case is isolated, and that doesn't mean they understand each other on that point. Like the rest of the Ancestors, this causes him not to accept Roa as one of the Dead Apostle Ancestors. When they meet again in Melty Blood Actress Again, Nrvnqsr remarks that the "name of the archangel has fallen indeed," and Roa calls him "pal." Nrvnqsr finds his incarnation as SHIKI to be the worst of his incarnations while Roa himself was cruder in the past. He compares his gradual assimilation with chaos with Roa's constantly diluting ego, commenting that it may be the last time they talk with one another. While Roa found perfecting the "Soil of Genesis" to be fun, and that the idea of working together on cooperative research is appealing, following their natural destructive urges, they decided to simply crush one another.
Characters berate him for attempting to act important during comedic scenes in Melty Blood whenever he acts serious. He is considered part of the Black Alley Alliance alongside Satsuki Yumizuka, Sion Eltnam Atlasia, Riesbyfe, and White Len, who mentions that he is even at the bottom of the group. Kohaku continues to call him "Mr. Nero" and "Mr. Neroa" in order to infuriate him during his time in the limelight. He does his best to assert that, while others may not know him, she should at least recognize him as SHIKI, but she comments that the SHIKI she knew didn't look like some pianist.
Roa has no interest in anything in particular ever since he was born, feeling more mechanical than human. His first feelings were those of pity instead of anger or despair. He simply thought of everything around him as pitiful, and himself as the most pitiful of all. He saw life which breaks down, daily life which fades, and time which is forgotten, all without a single limit. Though everyone falls, they still continue struggling desperately to continue their existence. When they judge that continued existence is impossible, they continue to reproduce instead, but even that dies in the end. He found that there is no recompensation, not for those who die, as even in the beginning they had no hope, but rather the noble wish and yearning for eternity which everyone longs for yet cannot achieve. The constant repetition of wishes is nearly impossible to measure, so he wished to replace the almost-cynical cycle with one pure thing. As he lived for one reason, which couldn't be termed as a "conviction," he sought the first word that he had happened to learn, the word "eternity." He believed that the prospect of eternity is the only goal worthy of effort. He wasn't interested in that which breaks down, and wished to see and understand death so that he could take a bit of interest in it.
While those seeking eternity often become Dead Apostles in pursuit of their goals of achieving it, it was simply a means of furthering his own goal for Roa. He believes nothing in existence has yet to reach the level of immortality, something that would define eternity. When something has shape, it cannot help but perish. Every living thing internalizes death from the moment they are born. Something about death within them is probably something that has never existed in the first place. Human vessels are incapable of resisting the notion of wearing down physically and mentally with time, and they do not have the capability to last one thousand years. Even a vampire's body does nothing to stop the aging of the mind, requires stealing from others to exist, and their supplemental parts must be from the same species, humans in general, meaning that they are not versatile. He calls their process degeneration instead of evolution, and identifies them as a species that ages slower and dies harder rather than any sort of embodiment of eternity. That which can't function independently in perpetuity cannot be called eternal, so depending on others to halt their aging makes it presumtuous to call them immortal. The only thing that was allowed by becoming a Dead Apostle was the ability to realize this goal with time that his human body wouldn't allow.
While aging can be stopped, that, in fact, is only halting it, and a stopped mind has no meaning whatsoever. Creatures that can function independently in perpetuity in a state close to perfection exist, such as beings like jellyfish that can feed on themselves and multiply, transforming old cells into nutrients to produce new cells. They have no such thing as a lifespan, but they can only be called perpetual due to their lack of intelligence. Eternity without having intelligence is the same as gaining eternity from death; even with a body that is unable to age, it can't escape death as long as there exists something. The only way to truly escape death is to die, creating a contridiction that cannot be solved. There cannot be anything in the world that cannot die. He instead concluded that preserving his existence after death is what would make him truly immortal.
Becoming eternal by living as a human with immortality is impossible, as long years will eventually destory the body and wear away the flexibility of the mind. He wasn't interested in such a stained immortality, as persistently maintaining individuality makes eternity unattainable. Instead of choosing immortality as a single human, he sought to choose an infinity where he could exist eternally. He devised his method of reincarnation to restart from birth, exist as himself, and die once more in order to preserve the purity of his own existence, a cycle that would preserve the purity of himself throughout the generations. He realized eternal reincarnation far better than anyone else, but it still requires new unborn children other than himself to continue. While he would lose his eternity with the extinction of the human race, he believes that "eternity only exists as long as there is an observer." He wouldn't be able to confirm himself if no humans existed, which would make continuing to live useless and bring his immortality to an end. While Nrvnqsr Chaos believes this is far from a true thesis of eternity, Roa believes that anything comes to perish should perish itself. Everything will become unchanging once observers cease to exist. As he cannot turn everything into nothingness, he wishes to act as an observer and live until that point.
Roa instantly fell in love with Arcueid upon seeing her for the first time, having such a feeling for the first and only time. Though it was only an instant, he lost his purity the moment his heart was stolen by her. She was someone who was not aware of her own self, and was only treated as a method of killing. He viewed her during her brief respite between killing her enemies and returning to a sleep that she cannot wake from herself. As she gazed at the moon with distant eyes while not knowing of her own fate, he saw her as the embodiment of the eternity he seeks. He was so pure that he could not understand his own feelings, misconstruing the act of thinking about someone else so much that it almost made him lose his sanity and go insane as his first feelings of hatred in his life. She changed him, and while she didn't change his fundamental goal of attaining goal of attaining eternity, she degraded it from seeking eternity itself for no reason, a pure, undefined will, into a means to an end of eternally reaching out for her. While he was able to understand this himself, he was, on the other hand, not able to do anything to change his heart. He regretted having lost his original purity because now, like all the other pitiful wretches, he wished to be able to continue existing as himself as he was right then.
Roa as an individual has ceased to exist, and the part of his soul left over can only be called the conceptualization of Roa's desire for immortality. He considered one's ego as unnecessary, so he doesn't really care what he does as long as he can continue his thesis on immortality. The only remaining emotion in his heart after eight hundred years and seventeen reincarnations is that first image of Arcueid. Even though his personality has long died out, it is an eternal memory burned into his soul that continues to exist without ever disappearing. Said memory was what caused him to hate the True Ancestor that caused his purity to fall, her existence, and everything else about her. His hatred guided him to trick her, become a Dead Apostle, steal her power, and wait for her to come to him. It has caused him to reincarnate so many times, just so he can wait for her to come chase him every time. Shiki believes that he shouldn't have made such mistakes if he was simply told that he was feeling love.
Roa actually has no direct control the minute he starts to possess Shiki. He only acts as a voice inside Shiki's head, but it often devolves into repeating nonsense as it urges him to act. Upon meeting Ciel, he repeats "Slice her apart" numerous times until Shiki uses his willpower to silence him.
Roa is the main antagonist of the "Near-Side Moon" routes, the stories of Arcueid and Ciel, of Tsukihime. His will has completely taken over during these paths, and he sets out to follow his usual goal of building his power as a Dead Apostle to lure Arcueid to him. SHIKI is the main antagonist during the "Far-Side Moon" routes, the stories of Akiha, Hisui, and Kohaku. SHIKI's inversion impulse keeps Roa from exerting his will over him, resulting in a very deranged individual obsessed only with getting revenge on Shiki for stealing everything from him. Roa doesn't have an appearance in these stories besides a brief mention from SHIKI that something in his mind wants to take over.
Due to the alterations to his memory by Makihisa, Shiki has completely forgotten about SHIKI's existence. He believes the two different styles of writing in the mansion was just his own childish way of being unfair to Akiha in their marking game, and he has reoccurring flashbacks to the moment where he was killed of which he cannot make any sense. He eventually starts to believe that he may have been the one to do the killing. Due to this, Ciel believes he is a very likely candidate for Roa's host, as all records of SHIKI have been removed in favor of portraying Shiki as the true son of the Tohno family. She interacts with him to determine if he has any relation to the recent incidents.
Roa in the Tsukihime manga is a mix of both versions of himself from the Far-Side and Near-Side routes. He bears an appearance similar to that of Far-Side route Roa, otherwise known as SHIKI, but contains the abilities, goals and the clothing of Near-side route Roa. He only appears four relatively small times in the anime, though his face briefly flashes across the screen during the opening. In the manga, he made four very large appearances, becoming progressively more dangerous each time as he begins utilizing continuously greater feats of Magecraft. During his first three appearances, he is always targeting Shiki so as to take vengeance for his death as SHIKI; Ciel manages to interrupt him just before he kills either Shiki or Akiha in every incident, although during the third time, he believes he has successfully killed Shiki by the time she arrives. During his fourth appearance, through the use of a Bounded Field, he manages to take control of quite literally the entire city of Miyaki's supply of Prana, directing it all towards his final battle with Arcueid.
During Arcueid's route, Roa doesn't make an appearance until after Nrvnqsr's death. Shiki initially believes that the end of the city's vampire had come with the defeat of Nrvnqsr, but he learns that the vampire Arcueid was hunting was someone completely different. Nrvnqsr was a completely separate vampire with the sole purpose of hunting her, while the original vampire that she came to find is still active. He learns small pieces of Roa's identity, but not of his true connection to Arcuied. Roa makes contact with Shiki after he parts with Arcueid during the night three days after Nrvnqsr's death. He engages Shiki with a knife, but he is left on the defensive until Shiki becomes distracted by his disoriented memories being brought back by SHIKI's face. Ciel eventually interrupts the fight by piercing Roa with three Black Keys and engulfing him in flames. Still silently cursing Shiki with his gaze, he is forced to retreat due to the attack.
During Kohaku's route, SHIKI is killed quickly by Akiha. Due to killing SHIKI with her abilities, some of his lifeforce, including part of Roa's soul, is transferred to her. This greatly increases her aggression, causing her own inversion impulse to hasten. She becomes extremely obsessive over making Shiki her own.
He makes his first appearance in Melty Blood:Actress Again as a playable character. In his own story mode, He is suprised to find himself alive after being killed by 'that woman'. He then wanders around until he runs into Len, where he apparently kills her and proceeds to use her to call out her 'master'. Shiki then arrives and the scene and after a brief exchange of dialogue, the two fight and Shiki is defeated, though Roa chooses not to kill him completely. After a battle with Ciel, Roa meets Arcueid who tells him that she will kill him once and for all, but disgusted by the fact that Arcueid has come to care for humanity, Roa calls her a 'fake' and fights her until somehow, he manages to awaken the princess-version of Arcueid. She is amused by his efforts and tells him that she will allow him one last 'kiss' before destroying him. Delighted at the sight of the "Eternity" he had witnessed 800 years ago, Roa proceeds, saying that he would gladly burn up his 18th and last cycle just for her.
He also makes appearances in other character's story modes as well, but as mentioned above, he is present as nothing more than a standing joke.
Roa makes a brief appearance in the Extra Season episode of Carnival Phantasm, during the Friends segment. He meets with Nrvnqsr Chaos, and they have a friendly talk with one another in an alleyway. Roa complains while he starts shedding tears that Akiha Tohno doesn't care about him and that Shiki has completely forgotten about him but nevertheless voices his happiness that he has Nero as a friend. His few other appearances are during the ending credits as well as at the party at the Ahnenerbe cafe at the end of the series.
Roa reached his peak as a human, so he sought to become a Dead Apostle to complete his magical theory, the thesis on immortality, and obtain Magic. Normally, the process to become a vampire would have taken at least one hundred years starting from the level of a Ghoul, but Roa wished to greatly speed up that process. When becoming a Dead Apostle by having a True Ancestor suck their blood, the power of the Dead Apostle raises in proportion to the True Ancestor's strength. He used the strongest True Ancestor, Arcueid Brunestud, in order to instantly become the strongest Dead Apostle by skipping the stages of growth with her immense power, and planned to rise to the top of the hierarchy in just ten years. Upon having her take his blood, he stole a large amount of her power, and managed to reach the apex of his magical abilities as he built up his power. Taking the place of a Dead Apostle Ancestor sealed by the Church, he gained their followers and rose in power among the twenty-seven, and managed to defeat Altrouge Brunestud in battle. He was eventually destroyed by Arcueid who had joined forces with the Church, but he had already managed to establish his system of reincarnation. While he is considered as an Ancestor by the Church, and Arcueid considers them to be a group of twenty-eight rather than twenty-seven, he is listed as "Extra" outside of the regular twenty-seven because they consider him to be a heretic who does not uphold the ideals of regular vampires.
Once his will is awakened in a host, he starts off as a weak Dead Apostle, and begins to increase his power by creating The Dead throughout a city like a traditional vampire. Draining the blood of humans, he increases his power bit by bit as more victims are slowly transformed. Due to being born as a human baby with parents, it is impossible to detect him beforehand. He shows no signs of being anything other than human, and only transforms into a vampire upon reaching an age where he can do everything at a satisfactory level. He quickly begins to utilize the relationships he gained until that time to blend perfectly into society, and it is usually only when the entire town is turned to The Dead, the Church is able to finally detect his presence. Due to the fact that he only chooses fine lineages and excellent hosts, it is much easier to for Ciel, who has innate knowledge of how Roa works, to find him by searching out the families in the area that meet his criteria. It quickly allowed her to find the bloodline he reincarnated into in Misaki due to the Tohno family being the only old family that meets his criteria.
He only acts one time in total himself to create a Dead, and then controls it in order to create even more to build his power and expand his territory. Of each victim they attack, more than half of the energy goes to him and the chance for another Dead to be born is created. While many are created, a number of bodies fail to awaken, leaving behind victims. During Tsukihime, while there are only ten known victims, the failures, the actual number of Dead number over one hundred. He hides his presence during this period, acting like a queen bee being nourished by the worker bees, to avoid being immediately found by Arcueid or the Church. He also uses various magical techniques to makes things seem not out of the ordinary even with the victims appearing. Due to utilizing only the Dead while he sleeps, it is hard to locate him before his power grows enough to make him a threat. Even if the Dead are targeted to lure him out of hiding, he can just send out a few to collect the minimum amount of blood to survive, making it a long, tedious process for Arcueid to hunt him. After working to build up his power in Tsukihime, Arcueid believes he is around the level of a "Class Five" vampire, but lacks the ability to become as strong as other Ancestors.
The status of being a Dead Apostle is stored within his soul. Those transformed by a True Ancestor don't just have their bodies corrupted, but also forever have their souls spoiled by the bite. Those he takes over return to normal if his influence is removed from them. Ciel, despite acting as Roa for a long duration of time, has no remaining traces of having been a vampire. Shiki, who is slightly taken over by Roa in Ciel's route, slowly gains the advantages of a Dead Apostle, such as strength, speed, and regeneration. After Roa is fully destroyed, his body returns to normal without any traces of having been changed. Arcueid's stolen power is also kept within his soul, and it will not be returned to her unless it is fully destroyed. Even after killing him seventeen times, she is unable to regain it until he is fully destroyed during the Near-Side routes. Only SHIKI is killed during the Far-Side routes, so Roa will continue on to his next reincarnation.
When Roa's manages to fully become the dominant force in SHIKI's mind during the Near-Side routes, he takes on the full benefits of being a Dead Apostle created by Arcueid. As a Dead Apostle created by a True Ancestor, being under the full moon greatly increases his power, and it can be said he is practically immortal under such conditions. He already has supreme regeneration from the deepest of wounds, and the effects of the moon only serves to increase it even more. He is able to regenerate his entire body after it is almost entirely destroyed by Arcueid's Marble Phantasm with the exception of his ankles. After the attack, his ankles start running as the rest of his body is regenerated with each step. He says that if the moon were waning even the slightest bit, such regeneration would have been impossible.
Roa's magecraft is Numerology, which is related to the Kabbalah’s Thaumaturgical Theory. He specializes in lightning based attacks, and most of his abilities involve electrical discharges. He makes use of the spells Set, a trap-type spell, Snap, a magic projectile attack that falls under the Single Action-type, and Square, a spell that amplifies the power of other spells. As one of the Dead Apostle Ancestors, he possesses a Reality Marble, Overload, which applies the effect of the Square spell to all of his spells while it is activated. Spells inherited by Ciel also show that he has knowledge in High Speed Incantation, Simplified Repetitive Incantation, Suggestion, Church Sacraments, and Necromancy. He has the ability to modify his appearance to suit his own preferences, such as when modifying SHIKI's body when he fully takes control, or briefly taking on his original appearance in Actress Again. During the manga, he creates a complex Bounded Field over Misaki after laying numerous magical seals across the entire town and using the school as the center for his lair. Much like Caster, he uses it to drain prana from the land and living organisms of the area, allowing both him and his lair to regenerate with the energy.
He has vast knowledge in other areas as well, such as spells forgotten from the Age of Gods, one of which, Soil of Genesis, he teaches to Nrvnqsr Chaos in the interest of helping him shape the chaos that makes up his existence. Nrvnqsr believes that Roa may have even had been able to find a method to completely control the chaos if he had managed to live, stating that it must have been regretful not being able to fully utilize such a clear heresy before being destroyed. He has great understanding of the True Ancestors and Crimson Moon, making him capable of discussing and analyzing Crimson Moon's methodology and plans for reincarnation with the aspect of him that exists within Arcueid. He states that his owh method of reincarnation is more efficient due to the fact that he only requires his base existence remaining intact, while Crimson Moon needs a vessel of the same level to revive. Roa doesn't use any magecraft within the events of the Tsukihime visual novel due to not yet recovering the proper knowledge to use it. He only makes use of it during his Melty Blood appearance and the Tsukihime manga.
As SHIKI, he is skilled with using a knife in close-quarters combat much like Shiki. He has Mystic Eyes with properties similar to Shiki's Mystic Eyes of Death Perception, and uses them to aim for lines when fighting with his knife. While Roa believes that they are the same Mystic Eyes of Death Perception that Shiki gained from viewing death, they are significantly different. His eyes allow him to see "the point that makes things live", making them more akin to cutting the "life" of a person rather than the "death". He cannot see lines on inanimate objects or destroy them, as it stems from the abilities of the Tohno's bloodline to perceive the "life" of a being and then "plunder" it. The lines seen by the eyes have the same placement as lines of death, allowing Shiki to block Roa's strikes by viewing the lines on his own body to anticipate the target of the cut.
During the Far-side routes, SHIKI's original personality is dominant over that of Roa, so he makes use of the body's original capabilities and so he lacks the regeneration technique of Near-Side route Roa. He uses melee attacks from both his knife and his own claws, he also has a skill called "Immortality" which instead of simply regenerating from heavy wounds, allows his body to reshape on it's own, so that he can survive even without vital organs. This also allows him to crystallize his blood, which then can be used as a extremely fine sword or thrown weapon (Roa proper does however utilize this ability himself in the manga). Additionally to this skill is he able to take in a body part of another person, by eating their flesh, and then use it as his own. This skill is called "Fusion" and is basically like an organ transplant without a limit. This ability also allows him to reattach lost limbs and to junction his lifeforce on to another living individual, and continue to live on, through that method.
Roa's method of reincarnation involves him having altered his soul, making him possibly the only person who has succeeded in turning the incalculable and unchangeable soul to a changeable and transmittable information. Normally, the soul scatters and returns to Akasha after leaving the envelope of the body upon death, but Roa's soul is able to be transmitted. It has lost the concept of destruction entirely and morphed so that it can remain conscious even while held within the record. No matter how many times his body is destroyed, he will always revive within a new one after a period of time. While it doesn't simply propagate through the air to take over the body of another, it can be called something similar to an electromagnetic wave. The human brain transmits and receives the soul constantly to ensure his continued existence.
Arcueid only has the ability to kill his body rather than his "meaning", so she has failed to completely destroy him. Only something with the concept of "rejection of reincarnation" like the Seventh Holy Scripture can disperse his soul into nothingness and stop his it from resurrecting, or it must be destroyed completely, such as by having his point of death pierced by a Mystic Eyes of Death Perception user. He cannot truly be called Roa as of his latest incarnations, as his own ego has started to fade away with each reincarnation. He is a malicious conceptualization made up of a steady accumulation of wills, history, and magical knowledge that has little in common with his original self besides his goals. Falling in line with his thoughts on an ego being unnecessary, it might have been part of his objective from the start. Among the Dead Apostles, he is known as the Serpent of Akasha and Ouroboros due to the serpent being the symbol of infinity and cycles. It suited him perfectly as something constantly shedding its skin and gaining a new body. The Church instead records his him as the Infinite Reincarnator.
Roa picks each of his vessels beforehand, choosing a balanced mixture both wealth and magical abilities to ensure his quick ascension to power without attracting attention. Being born into a family of high social standing, property, and money makes it easier to make the whole town into vampires as he pleases. The most important aspect is having special abilities from birth. Magecraft is something that can be learned, but having inborn abilities passed on by blood allows for greater power. He only chooses those with "something not human" in their lineages, such as those called Demonic Children or psychics. With wealth, influence, and inhuman power, he is able to follow his desires each time he awakens.
He only needs the bloodline itself, allowing for him to set reincarnation process while the vessel isn't even born. He can also have two active vessels at one time, such as Elise and SHIKI, allowing him to quickly move to the next without waiting for a long period of time. When the vessel is born, all of the "information" of his self, his soul, is transmitted into the body. The transmigrated soul is stored deep within the baby until it matures or gains enough intelligence of its own succeed "his self." While the person is definitely has the same soul as Roa, they are initially raised as an entirely different person, under a social community, growing up physically and mentally until Roa's intelligence is awakened. The incarnation speaks with whatever dialect the host uses, so in the case of SHIKI, sometimes casual speech such as "pal" is used. He finds the use of different dialogue to be refreshing and part of the fun that comes with a new vessel.
Once awakened, the will of the host no longer truly matters. Whether the environment raised them to be a good or bad person, they instantly start to adhere to Roa's will and become a new vampire that is Roa. It isn't the same as two people sharing one body, as the human is considered to be Roa. While Ciel was initially feeling his will, she was disgusted by the thoughts. She gradually fell into its influence, and even began to enjoy the gruesome slaughter of everyone dear to her. The physical characteristics are based on the body, though he may change some aspects to suit his own preferences. The full personality is definitely Roa no matter how the person was during their life, displaying Roa's coldhearted, thorough and selfish, yet gentle and intellectual nature.
During every cycle, he was able to retain all the memories of his previous incarnations. As a consequence, since they all experienced the same things Roa has, their general opinions remained the same. His last reincarnation (the 18th) is SHIKI Tohno, and his previous one was Elesia (Ciel), who he considered his most powerful one other than the original. He had reincarnated 17 times, but he never reached the potential of the original Roa again. This is because none of his incarnations ever obtained the same incredible amount of Circuits he did and, even though he technically possesses every memory his past incarnations had, he draws these memories only gradually from the Akashic record, so he is forced into waiting for the specifics of his former lives to reappear to him. This is the reason why he is unable to use Magecraft during the events of Tsukihime; he simply no longer recalls exactly how to do so and must bide his time till he is able.