Walter C. Dornez was a butler to the Hellsing family before he betrayed them and joined Millennium as a Vampire.
Walter is captured by The Captain during their invasion of England. He has gone through a similar process to the Major and the Doc's's troops (although hastily conducted, according to the Doctor), and now appears thirty or so years younger, serving as Millennium's new, brainwashed, secret weapon (when the Major sees the finished product, he remarks "It was determined 55 years ago that you would join us"). Whether or not this brainwashing is permanent remains to be seen. It is also currently unknown whether or not Walter's brainwashing by Millennium was consensual, or if he was brainwashed against his will. In the one of the most recent chapters of Hellsing, Walter has leaped from his perch on one of Alucard's stakes to confront him, crushing what was left of Alexander Anderson underfoot, saying that people become trash once they are dead, and that trash should not be mourned for. He then tells everyone he was brainwashed and rejuvenated. Enraged by his desecration of Father Anderson's remains, Yumie attacks him, but is easily outmatched and cut into pieces in a single move.
After listening as Walter explains himself, Alucard criticizes his decision, saying that his old, aged body was many times more beautiful than his new, unnaturally youthful one, referencing the hasty and flawed process that posed. Also, during Walter's brief fight with the Captain, the Major's dialogue seems to imply his previous remark meant he had made the choice to forcefully draft Walter into Millennium ever since witnessing his feats of power during WWII.
Formidable as his new form was, the rushed and flawed transformation procedure rendered Walter's body highly unstable. This manifested by his growing increasingly younger until he resembled his fourteen year-old self. At this point, he could no longer withstand bodily harm, and later his right arm crumbled apart.
In the TV series, Walter is never artificially rejuvenated, instead saving Integra and then crashing in a helicopter. He later appears on a stretcher and mentions Alucard as "the true immortal vampire." However, it should be noted that the TV series does not follow the plot of the manga, as the Major and Millennium do not exist in it, despite the fact a svatzika can be clearly seen on the artificial vampires' microchips.
In chapter 75 of the Hellsing manga, Sir Hugh Irons, the acting leader of the Convention of Twelve, comes to the conclusion that Walter had in fact been betraying his superiors for quite some time, even before the second coming of Millennium and possibly even since the days of the World War II. He mentions that he foresaw Richard Hellsing's attempted coup of the Hellsing Organization and also Richard's assassination attempt on Integra, and had quite specifically warned Walter of it, charging him with protecting her; and yet, despite his warning, Walter was nowhere to be found on that day. Therefore it is suggested that Alucard's awakening was also orchestrated by Walter; his convenient absence left Integra with no other choice but to release Alucard, whom Arthur Hellsing (Integra's father) had purposefully sealed away, thinking him to be too dangerous to use as a weapon.
Other bits of evidence suggesting that Walter is a traitor include the Valentine brothers' mysterious knowledge of the location of the Hellsing mansion, Walter's suspicious memory lapse when Millennium resurfaces (he does not seem to remember storming and destroying Millennium's base in World War II, but claims that he does after Alucard prods him), and the strange omission of the fight scene between Walter and his apparent rival, the Captain. It may be that, after seeing Alucard's abilities during World War II, Walter wished to destroy him to prove himself and thus allowed Alucard to awaken. When Integra demands an explanation for Walter's betrayal, he does not provide one, and simply demands that she no longer address him as Walter. Alucard then mentions that Walter's new body is nowhere near as beautiful as his old one, addressing him as Shinigami. He then asks him if this transformation of his was undergone with the desire to become "a Shinigami (angel of death) in body, mind, and soul", and Walter confirms this. Integra then reluctantly but resolutely orders Alucard to kill him.
As Integra and Seras leave to go after the Major, Alucard teases Walter about being the only servant to his mistress (Integra) and having a servant of his own Seras), and that the love of neither can be Walter's again. In chapter 78, the battle between Walter and Alucard begins. Alucard lunges at Walter amidst a flurry of bats, reaching out to grab his face, but Walter easily severs Alucard's arm with his wires. The wires then wrap around Alucard's leg, and Walter tosses him into a building. Alucard then attempts to shoot Walter with the Casull, but Walter deflects the bullets with square, mesh-like barriers, formed from his wires. Alucard summons his hellhound, which Walter calls Baskerville, and it is sliced in two. Finally, Alucard aims the Jackal at Walter, but it explodes and maims his remaining hand; Doc is shown holding his remote control, which has a button marked "JACKAL". Although we do not know exactly how long Walter has been betraying Hellsing, we can confirm that Walter's betrayal occurred before the beginning of the manga storyline, since Walter gave the Jackal to Alucard in volume 2.
On the last page of chapter 78, Luke Valentine emerges from the remains of Baskerville.
In chapter 79, Walter, using his wires like puppet strings, gains control of both Alucard's Hellhound and Luke Valentine, and uses them to attack Alucard. Alucard makes short work of the two however, and we are then shown Walter falling to the ground and coughing up blood. His wires, for the first time, go slack. It is implied that Walter's time is short as his conversion was a rushed and flawed process. In chapter 80, Walter summons up the last vestiges of his strength and uses his wires to sever Alucard's arms and legs, to cut his torso in half, and to suspend him helplessly in the air. Walter once again falls to the ground, seized by pain. When he stands up, his age has regressed, and he appears as he did during Hellsing: The Dawn, as a 14-year-old boy.
Walter then grabs a piece of rebar and attacks Alucard, impaling him through the chest. It is revealed that the Alucard that Walter had attacked was a decoy, created by Alucard from Luke Valentine's remains. Luke Valentine (supposedly) dies, and Alucard appears behind Walter. Alucard now appears in the form as he did in The Dawn, although the reasons for this are as yet unclear (it is assumed that he assumed his form on his own accord, perhaps to psyche Walter out). Alucard then punches Walter, and now appears to have the upper hand. Alucard goes on to present what he believes is the reason for Walter's betrayal; his fear of becoming old and useless. To prove his abilities to himself, Walter desired to destroy Alucard, and allowed this obsession to consume him.
The battle continues with Alucard releasing his full strength, drinking up the blood of the entire city. As he does so, the manga spends a few chapters with other characters. Upon chapter 87, Alucard brags that he is victorious; however, behind Alucard Schrödinger commits suicide, and drops his body into the blood that Alucard is drinking, the Major having stated that he planned to poison Alucard. After Alucard brags about Millennium's inability to defeat him, a voice states, "No, you lose, Alucard." Walter knows of this, and begs the Major to stop, launching one last attack on Alucard and slicing him in half from groin to head. Alucard simply laughs and continues to absorb the city's deceased inhabitants. The Major then explains to Walter that, despite his skill, his abilities simply are not sufficient enough to actually compromise Alucard's health. He goes on to say that, because Walter gambled everything he had on this one attempt to kill Alucard, his failure to do so has drained his existance of both value and meaning. Walter seems to take this to heart, as he stops his attack on Alucard. Alucard then grabs his shirt collar and mocks both him and Anderson for their respective failures as Walter simply stands there.
In chapter 90, after witnessing Alucard's defeat and disappearance, Walter is left alone on the battlefield, with his transformation eating away at his body. As he stands there, he grabs Alucard's necktie as a gust of wind carries it by, and laughs in artificial triumph over Alucard's apparent death. As he laughs, he vomits up blood again, and shouts "Is this really.... what I wanted!?" Shortly after, however, he's sniped and shot by Heinkel Wolfe from atop a building further away, in retaliation to him killing Heinkel's partner, Yumie, and desecrating Father Anderson's remains. Walter does not seem to care however, even telling Heinkel to keep shooting him. She gladly obliges, shooting him multiple times including a hit in the left eye. until she runs out of ammunition and charges at him head on. At this point, Walter seems to have a change of heart, as he tells her that she can beat him, but she will not be the one to kill him. After casually depriving her of an arm and a leg, Walter walks a short distance, bleeding profusely, and sends two strands of his wires into the grounded Millennium zeppelin. In chapter 93, Walter is seen in Doc's lab sitting on the floor with his left eye regenerated. He tells Doc he doesn't know when to give up and that he, and everything he's created, is good-for-nothing; he includes himself in that statement.
In chapter 94, Walter tells Doc that all of the actions of Millenium have culminated in the cheap comedy that the evening has become. He says that all of it will come to an end, and he tries to stand up. He succeeds, but his right arm dissolves. He mentions that it appears that Alucard was right about his new body, and says in a tired and mildy amused way that he appears to be comming to a horrible end. Doc then rants on about how his research will some day lead him to surpass even Alucard, but Walter simply says "rubbish" and that they will all die. Doc then tries to kill Walter with his remote, but Walter simply slices off his arm and leg, depriving him of the remote.
Doc falls, but in mid-fall he tears off the curtain covering the SHI and it unveils the skeleton of Mina Harker. Walter then reasons out that those remains have been the basis for all of the Doc's work, explaining that because Alucard (Dracula), was not killed, Mina was not cured of Alucard's curse, and a part of Alucard existed inside her. Mina was exhumed by the Doc, and researched until her remains were nothing but a ruin. Walter then tells Doc that all he has ever done was to bootleg copies of Alucard's imitation (Mina), reinforcing his position that the Doc and all of his work are but a cheap comedy.
The flames of the burning zeppelin engulf Mina's corpse, and Doc is killed by a support beam falling from the wreckage of the zeppelin. Walter then reflects good spiritedly on how much he wanted to defeat Alucard, closes his eyes in a similar manner to Alucard, bids farewell to Integra as Alucard had done, and at last, as an image of his aged human body is depicted, smiling and relaxed, he is engulfed by the fires of the zeppelin. His passing is sensed by Integra and Seras, the former shouting out for him as he dies. He is momentarily grieved for by both, and then they leave the zeppelin as it crashes in a manner reminiscent of the Hindenburg disaster.