Kotaro Yanagisawa

Shiro is a mysterious man and former caretaker of Itona Horibe.

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.

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.

Trivia

 * His clothing has shown to be of the same material that the 'Anti-Sensei' weapons are made from, as Kurosensei 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.