Kotaro Yanagisawa

Shiro is a mysterious man and former caretaker of Itona Horibe. He is a recurring antagonist to Class 3-E.

Appearance
Because Shiro wears white robes made out of anti-sensei materials and a mask over his face, his actual appearance is unknown. However, when he removes his mask in Chapter 133, he is revealed to have short, wavy black hair and light eyes with his left eye being a prosthetic eye.

Personality
During his first appearance, he seems almost funny and easygoing, but as he continues to manipulate Itona into attacking Korosensei, his true colors were revealed to be much more conniving and manipulative. Furthermore, he sees practically everyone other than himself as disposable (a trait that he retained since being a researcher). In his past as a researcher, he was revealed to be quite arrogant, amoral, and abusive given how he was willing to use human guinea pigs for his research and constantly beat up Aguri Yukimura, his fiancee, and even fellow scientists for supposedly getting in the way of his research.

History
Shiro's real name is Kotarō Yanagisawa, chief scientist of a project that involved trying to create anti-matter without massive energy requirements by generating it in a living body. He is also the heir to a wealthy family and a genius biologist in charge of the world research foundation, but was also revealed to have no ethics or morals in regards his research whatsoever. At some point Yanagisawa enters into an arranged relationship with an aspiring teacher Aguri Yukimura, however he only did so out of pity for her, and essentially treated her like a slave.

Needing a human guinea pig to start his project, he managed to gain custody of an imprisoned assassin nicknamed the God of Death, who was sentenced to death after his capture. Yanagisawa explained the reasoning as to why he did this, also stating that the inmate should be grateful that he at least has the chance to live longer. However since none of his colleagues wished to go near him due to the man's reputation and ability to brainwash people into doing his bidding, Yanagisawa chose Aguri to closely observe him, seeing her as expendable if the man somehow takes her hostage. For a year, the scientist kept his distance from the assassin but continued working on his project.

While the experiment appeared to be going smoothly, Yanagisawa and the other scientists were concerned with what would happen to the anti-matter cells if the living host were to age and die. As a precaution he sent a lab mouse injected with the same cells as the God of Death and a capsule to the moon to observe any changes. In March the mouse did die but the cells began to multiply and turn into matter. This mixed with the anti-matter of the moon and caused a giant explosion that wipes out 70% of the moon, giving it its permanent crescent moon fixture. Horrified that the same thing to happen to his human guinea pig in the following year, he immediately ordered preparations to have him killed. Aguri overhears this and warns the assassin with a plea to not die, which Yanagisawa watches through the cameras. Thus the assassin begins his transformation and escape plan.

He later survives the lab explosion when the assassin, now a tentacled monster, escapes albeit not without injury. His fiancee however did not. When Yanagisawa hears news that the assassin is currently teaching Class E at Kunigaogaka Junior High School (the same class and school Aguri was teaching at) under the name of Korosensei, he used a young boy named Itona Horibe and implanted him with the same tentacles Korosensei has.

Plot
Shiro makes his first appearance alongside Itona in Chapter 29 (Episode 11 in the anime), the latter being the second transfer student to Class 3-E. After Itona reveals that he is Korosensei's brother, Shiro clarifies this in terms of having the same level of strength and power, and also the fact that Itona has the same tentacles as Korosensei. When the two began fighting in an arena-styled area, Shiro called out each of Korosensei's weaknesses and also aided in the battle by emitting a special beam that paralyses Korosensei. However Shiro did not take into account the numerous anti-sensei knives the students own, and Korosensei manages to win the battle. Shiro takes the defeated Itona away, though vows to return to kill their target.

The two return for a second attempt after Takaoka's short stint as PE teacher, this time recruiting and manipulating the alienated Terasaka into preparing their next ambush. Shiro had Terasaka weaken Korosensei's defences, first by letting off a smoke can which caused the target to get a runny nose and then lace the pool with chemicals designed to weaken the power of the tentacles. The two successfully ambush Korosensei whilst rescuing the students and strike with his defences and attention down. However the class strikes back and prevent Itona from attacking Korosensei further, prompting Shiro and Itona to retreat.

Shiro and Itona return for a third time during the second semester, this time Shiro personally taking measures to discredit and frame Korosensei and cause the class to lose trust in their teacher. The two make their third strike on Korosensei using the strategy devised by the class at the summer vacation. However, Korosensei's improvements and the repetitiveness of Itona's attacks leads the former to easily win for the third time. When Itona starts to lose control of his tentacles, Shiro turns his back on the boy and abandons him. At the behest of the Ministry of Defense, Shiro resorts to a more direct approach, using the unstable Itona and Korosensei's duty of care towards his students to bait Korosensei into a trap. The students however fight back in revenge for Shiro using them and thwarts his men in succeeding his plan. Afterwards Shiro leaves to rethink his strategies but vows once more that he will kill Korosensei. Speaking with the Ministry of Defense, he states that among the class, there is one student who has the capability and talent of being an assassin.

Later chapters reveal that Shiro would head up the research in the ultimate weapon to assassinate Korosensei, and is given custody of the defeated God of Death. Shiro later returns in Chapter 127, elaborating in more detail about his opinion on which student has the best skills for assassination. When it was revealed that the student in question is Akari Yukimura who infiltrated Class 3E as Kaede Kayano, he stated that he had discovered this some time ago and had known that she stole his research. Having witnessed Kayano's almost suidicial assassination attempt, Shiro appears once more this time with his face fully revealed, and his left eye covered up, accompanied by a hooded hit man before leaving. Korosensei immediately recognises Shiro to be Kotaro Yanagisawa.

Itona Horibe
Shiro found Itona alone on the street after the boy was abandoned by his parents. He tapped into the boy's thoughts and want for strength, and soon began the procedure to implant tentacles on the boy. Despite being the caretaker, Shiro was ultimately using the boy for his own ends, essentially the killing of Korosensei, and the successive defeats and inability to get results eventually led Shiro to coldly abandon Itona.

Aguri Yukimura
Aguri was Shiro's fiancee before the explosion. Their relationship was an arranged one and Shiro only engaged with her 'out of pity'. He was quite abusive with Aguri, hitting her constantly and pressuring her to give up her teaching job to work for him exclusively. In turn, Aguri does not love him either but puts up with him as it was the duty of a wife to support her husband-to-be.

Akari Yukimura
As Kotaro, Shiro acted friendly to his wife's kid sister Akari (later taking the name of Kaede Kayano) and played magic tricks for her benefit. However Akari's experiences with different directors during her acting career led to her not trusting Shiro and attempted to have Aguri end the relationship with him. He eventually discovered that his sister-in-law took his research after his fiancé's death and infiltrated Class 3-E under a fake name, detecting a high level of bloodlust emanating from her after implanting tentacle seeds onto herself. He kept silent however and advised her to be more patient and wait until his ultimate preparations were complete but otherwise treated her in the same way as he did with Itona. Shiro's opinion was that because of her prodigious acting skills, he believes Kayano to be the best qualified to become an assassin.

Trivia

 * His clothing has shown to be of the same material that the 'Anti-Sensei' weapons are made from, as Korosensei was unable to touch him without the offending area being destroyed.
 * He has been shown to have a device which emits a beam of light that paralyses Korosensei for a short period of time. Whether this works of Itona is unknown.
 * He seems to be quite skilled at doing magic tricks.