Gaku Yashiro is the main antagonist of the manga and anime series Boku Dake ga Inai Machi
As a child, Gaku Yashiro was obsessed by things like psychology and people's reactions to specific situations. As he grew older, Yashiro became bored with his everyday life, where nothing interesting ever happened.
One of Yashiro's favorite books was "The Spider's Thread," a tale of a man named Kandata who, after falling into hell, was given a chance to climb out on a spider's thread. On his way up, Kandata kicked down any other sinners who tried to climb up with him, claiming it was his, and only his, chance to escape. The thread then snapped, plunging Kandata back down into hell for eternity.
One day, Yashiro's abusive older brother convinced him to lure a young girl to a shed, at which point Yashiro stood guard as his brother raped the girl. Yashiro then used kind words and presents to manipulate the girl into remaining silent about the incident.
Yashiro and his brother made the process a monthly occurence, where Yashiro's brother would get to rape young girls monthly, while Yashiro himself would get the "thrill" he wanted out of his life by getting the chance to manipulate the girls, which also appealed to his intrigue with psychology.
One day, Yashiro was approached by one of his classmates who had accidentally come into possession of a box of baby hamsters. Yashiro told her he would deal with the hamsters for her, at which point he went home, dumped them into a jar of water, and left them to drown.
Returning hours later, Yashiro was thrilled to find one of them had survived, decided to keep it as a daily reminder of the incident and the thrill he experienced, and named his new pet "Spice".
During one of Yashiro's brother's monthly rapes, due to Yashiro's own negligence, the boys' mother came very close to the shed where the rape was happening, and Yashiro's brother, attempting to keep his current victim from screaming, accidentally strangled her to death.
Yashiro and his brother tried to revive her, to no avail, so they hid the body, though Yashiro quickly realized his brother planned to frame him for the murder.
Yashiro then realized he could now see a spider's thread attached to his older brother's head, resembling the same thread from his favorite book.
At some point, Yashiro's hamster died, and while Yashiro felt a sense of loss, it wasn't out of care for his pet. It was because he now had nothing to entertain him in his day-to-day life.
Refusing to be framed by his brother, Yashiro planned to murder his sibling, then make it look like the boy killed himself out of guilt for raping and murdering the young girl from before.
Succeeding in his plan, Yashiro found the new "thrill" for his daily life: Getting away with murder and manipulating innocent people.
Everytime Yashiro found a "victim", he would visualize a spider's thread attached to their head, and, when he murdered them, it was as if he was "cutting the thread."
At one point, Yashiro noticed he himself had a thread, symoblizing his own vulnerability.
Beginning his spree
Decades later, Yashiro became an Elementary School teacher, and began trying to lure neighborhood children to isolated areas so he could murder them, however something would often go wrong, ruining his attempted killing. Yashiro would note these failures mentally in order to become better at his plans.
Becoming engaged to a child psychologist, Yashiro used her knowledge to gain a better understanding of children, which ultimately lead to him beginning his killing spree.
When confronted by his fiance, who believed Yashiro had something to do with a recent child death, Yashiro pushed her out of a window and made it look like she killed herself from too much stress.
Yashiro continued his killings over the following years.
Becoming a teacher at Mikoto Elementary School, Yashiro targeted the 10-year old girl Kayo Hinazuki, who was regularly abused by her mother.
Kidnapping Kayo, Yashiro took pictures of her half-naked body, threw her into a freezer, then sprayed her with a water bottle until she slowly froze to death.
Yashiro later kidnapped and murdered Aya Nakanishi, a student at a different elementary school.
Realizing the police are closing in on him as a suspect, Yashiro murders Hiromi Sugita, a student of his who looks like a female, knowing that, since the police believe the killer solely targets girls, they will believe the true murderer mistook Hiromi for a girl, effectively eliminating anyone who personally knew Hiromi was male from their suspect list.
Yashiro then successfully pinned the murders on a local food delivery worker, Jun "Yuuki" Shiratori, getting the boy arrested and put on death row.
2006 Framing of Fujinuma
By 2006, Yashiro had not only married the daughter of a politician, then killed said politician to steal his position as County Lawmaker, but had also murdered dozens of children over the years, always framing someone else for his crimes.
During one of his attempted kidnappings, Yashiro was spotted by Sachiko Fujinuma, a reporter who investigated the 1988 kidnappings, and the mother of one of his former students.
Ditching his planned victim, Yashiro, knowing Sachiko could incriminate him, follows her home, where he brutally stabs her to death, and gleefully watches as she slowly bleeds to death.
Waiting for the woman's son, Satoru, to return, Yashiro frames the young man for his mother's murder.
Satoru, trying to avoid arrest, is flung back in time to 1988 by his "Revival" ability, and is given the chance to stop Yashiro's serial kidnappings.
Back in 1988, Satoru attempts to foil Yashiro's plans, and, while prolonging the lives of Kayo, Aya, and Hiromi by a few days, he is unable to save them from death at Yashiro's hands.
Back to the present
Returning to 2006, Satoru, with the help of his teenage co-worker, Airi Katagiri, avoids the police chasing him for his mother's murder, and further researches the crimes Yashiro has committed over the years.
Yashiro, now going by "Nishizono Manabu", leads the investigation trying to capture Satoru, and, in an attempt to further incriminate Satoru, tries to roast Airi alive by setting her house on fire while she is inside.
Satoru rescues Airi, and, after being tracked down and arrested by the police, is once again sent back to 1988 for one final attempt to save his friends.
Satoru's Final Chance
Back in 1988 once again, Satoru uses his knowledge of the past to successfully save the 3 children this time around.
Believing The Murderer will next target a young girl named Misato, Satoru seeks help from his homeroom teacher, Gaku Yashiro, in pursuing her in his car.
Yashiro then reveals the horrifying truth that he is the real killer to Satoru, and, after gloating about it to him and claiming murdering is an act to "fill a void of happiness in his everyday life", locks him in his car, then pushes said car into a frozen lake, hoping to drown Satoru.
Satoru survives this attempt on his life, though he is plunged into a coma from which he may never wake.
Due to Satoru's saving of the children, all of his friends grew up to live happy lives.
Yashiro, meanwhile, grew bored with killing children due to how "thrilling" his rivalry with Satoru was compared to ordinary police, and as a result, eventually stopped murdering children due to how dull it became. However, this didn't stop him from murdering anyone who got in his way throughout his life.
Satoru Wakes Up
Yashiro, now going by Nishizono Manabu, hears that, after 15 years, Satoru has finally woken up. Thrilled at this, Yashiro begins stalking Satoru and his new friend, Kumi, a young girl who is a patient at the hospital Satoru is recovering at.
Using his political position, Yashiro arranges an outing to nearby campgrounds for every patient at Satoru's hospital, and, disguising himself, attends the trip as well.
Attempted Murder of Kumi
Arriving at the campgrounds, Yashiro steals Sachiko Fujinuma's cell phone, and uses it to text Kumi to meet "Sachiko" at a nearby storehouse, where he seemingly plans to murder her and frame Satoru for the crime.
This plan fails, however, as Satoru and his best friend, Kenya Kobayashi, having learned of Yashiro's intentions, rescued Kumi before Yashiro could get his hands on her, effectively leading him into a trap.
Yashiro's true plan revealed
However, this was all part of Yashiro's plan, as he tricks Kumi into drinking a drugged drink that puts her to sleep, at which point he puts her on a slowly-sinking canoe drifting along the river.
Yashiro then plans to lure Satoru to a bridge where he will have a clear view of Kumi drowning, at which point Yashiro will set the bridge aflame, killing Satoru and dooming Kumi, while simultaneously removing any and all evidence of Yashiro's villainy.
Satoru, however, realized Yashiro's plan, and confronts him on the bridge before he has a chance to set the explosives.
There, Yashiro taunts Satoru about both his inability to be catch him, along with Kumi's inevitable death.
Satoru then reveals not only that Kumi has been saved by his friends, but also that the reason he's been a step ahead of Yashiro for so long has been because of his "Revival" ability.
Yashiro, realizing he's been caught and refusing to be arrested, sets the bridge on fire while both himself and Satoru are on it, hoping to kill both of them in one fell swoop.
Satoru, however, tackles Yashiro, sending them both into the river below, where they are both saved by nearby law enforcement.
As Yashiro is arrested, he sees, through his reflection in the river, that his own "Spider Thread" has been severed, signalling that he has been truly and completely beaten.
Kenya visited him in the prison after confessing his crimes include murders of his older brother, fiancée and his father in law.
- He is voiced by Mitsuru Miyamoto.
- As a child, Yashiro manipulated young girls to gauge their reactions and observe their psychology, but as an adult, he murders children, frames innocents for it, and manipulates others solely for the "thrill" it brings him.
- He is very similar to Mark Jefferson. They are both the culprits in a time-travel based mystery, they're both the protagonist's teacher, and the viewer is made to believe that they're good people trying to help the protagonist.