Kotaro Yanagisawa

Shiro (real name Kotarō Yanagisawa) is a mysterious man and former caretaker of Itona Horibe. He is a recurring antagonist to Class 3-E.

Appearance
Shiro wears white robes and a mask over his face, all made from anti-sensei fibers. His actual appearance was unknown until he removed 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. This chapter also revealed that he used voice changing technology to hide his identity from Korosensei.

Personality
During his first appearance, he seems almost funny and easygoing, and although he retains this attitude while he and Itona attack Korosensei, it soon becomes clear that he is incredibly conniving, manipulative, and presumptuous. Furthermore, he sees practically everyone other than himself as disposable. In his past as a researcher, he was revealed to be quite arrogant, amoral, and abusive; he was willing to use human guinea pigs for his research and constantly takes out his anger on his coworkers and subordinates, including his fiancee Aguri Yukimura, for supposedly getting in the way of his research. Because of his personality, Koro-sensei exploit it twice for his advantage before he met his demise.

History
Shiro's real identity 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. He himself admitted that he lacked the passion. While still a university student, he met aspiring teacher Aguri Yukimura, whose family ran Yukimura Pharmecuticals. As the company struggled to keep afloat, Yanagisawa bought the business and entered into an arranged relationship with Aguri. The real reason was that Aguri's passion for teaching would offset his lack of passion in his chosen field. Outwardly however, he stated that he only did so out of pity for her, and essentially treated her like a slave devoid of rights.

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. The result was a catastrophe: 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 will happen to his human guinea pig in the following year, Yanagisawa immediately ordered preparations to have his test subject terminated. 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. The disastrous failure of the project, his guinea pig's 'insurrection', and his property's change of heart profoundly rendered Yanagisawa's pride to shreds.

Yanagisawa later survives the lab explosion when the assassin, now a tentacled monster, escapes albeit not without permanent loss of his left eye. 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.

First Semester
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.

Second Semester
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 at the end of the second semester, 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.

Final Semester
In February, Shiro (now unmasked) witnesses his hooded accomplice revealed to be the faux God of Death destroy a thirty story building in a few seconds. While watching on, he reminiscences of how he was once a genius researcher but lacked the passion, and became Aguri's fiancé as a way to offset his flaw. But he lost it all when the moon was destroyed and Korosensei escaped. Over time he and the God Of Death vow to kill their 'teacher' by the end of March.

With 90 minutes left for the satellite weapon to fire its laser on Korosensei, Shiro and the God of Death penetrate the barrier preventing their target's escape. Upon confronting Korosensei, Shiro reveals the faux God of Death's top speed of Mach 40, and boasts about how superior he is to Korosensei. He then injects himself with tentacle cells, and joins to attack Korosensei. When Korosensei begins to have a degree of success in the battle, he abruptly switches plans, ordering the faux God of Death to attack the students. Korosensei takes several hits before Kayano attacks the faux God of Death, who subsequently impales her with a tentacle on Shiro's orders.

Shiro laughs insanely at her attempt to protect Korosensei, saying how she also died in front of him just like her sister. He watches with anticipation as Korosensei's skin turns black, signifying him giving in to his rage. Shiro comments how the colour black represents chaos and destruction and is pleased at this event. Using his final trump card, Shiro injects the God of Death with a stimulant to increase its power farther than that of Korosensei. As it moves to impale Korosensei, the teacher blocks the attack with a white light, shocking Shiro with this sudden change. He watches as Korosensei changes his colour with every emotion known, creating pure white energy. Korosensei then unleashes the energy in a powerful blast that tears apart the faux God of Death and sends Shiro flying backwards. Shiro is confident that the cells he injected himself with will protect him... Until he looks up and realizes in horror that he is flying straight towards the anti-tentacle barrier. With no way of stopping himself, he tries to strengthen his vitals just to survive but doubts it will help him. He begins to frantically think up of ideas to save himself but all ideas won't save him in time, leaving him doomed. His mind begins to break down at the fact that his death is at arm's reach and pitifully cries out in denial before passing through the shield, disintegrating him to nothing.

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
Main article: 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 (although she eventually acted without him). Shiro was less involved with Kayano's assassination attempt than he was with Itona's, and he was also notably harsher to Kayano than to Itona. Regardless, Shiro's opinion was that because of her prodigious acting skills, Kayano was the best qualified to become an assassin.

Trivia

 * His clothing is made entirely out of Anti-Sensei Material.
 * He also has a special beam of light equipped in his outfit and prosthetic eye that immobilizes Korosensei.
 * He seems to be quite skilled at doing magic tricks.
 * The symbol on the back of his cloak is a Bohr model of hydrogen.
 * Because he and Aguri never married, he is not officially Kayano's brother-in=-law despite stating himself as such.