Kotaro Yanagisawa

"As a result of a humiliating miscalculation... I lost my honor... and my woman. And it was all because of the original Grim Reaper. All because of you..."

- Yanagisawa desiring vengeance

Kotaro Yanagisawa (柳沢 誇太郎 Yanagisawa Kotarō), initially known as Shiro (シロ), is a mysterious scientist responsible for creating the tentacles, which establishes him as the main antagonist of the series.

Once a proud scientist, Shiro wants to kill Korosensei for taking advantage of his experiments and ruining his life two years ago. Shiro is an influential character, as the repercussions of his research make him both directly and indirectly responsible for many major events, including the destruction of the moon and the creation of Korosensei.

Appearance
While in costume, Shiro wears white robes and a mask over his face, all made from anti-sensei fibers. Additionally, the back of his torso is adorned with a Bohr model of the hydrogen atom. Shiro also utilizes a voice modifier to further hide his identity from Korosensei.

When unmasked, Yanagisawa has short, wavy dark green hair and pale teal eyes, along with fair skin. As a result of his confrontation with Korosensei two years prior to the events of the story, a prosthetic eye replaces his left.

After injecting himself with a modified version of the tentacles, Yanagisawa discards his mask and top robes, revealing a black tank-top underneath. Dark veins sprawl across his entire body and the muscles in his arm bulge to shape themselves like tentacles, allowing him to conceal a syringe inside them. Additionally, in the manga, the sclera in his right eye becomes completely black; in the anime, his eye simply becomes paler.

Personality
During his first appearance, he seems funny and easygoing, and although he retains this attitude while he and Itona attack Korosensei, it soon becomes clear that he is conniving, manipulative, and ruthless in his ambitions. Furthermore, he sees practically everyone other than himself as disposable. A notable example was in his past as a researcher, where he was revealed to be quite apathetic; he was willing to use human guinea pigs for his research and took out his anger on his coworkers and subordinates, including his fiancée Aguri Yukimura, for getting in the way of his research.

Yanagisawa's sadism intensifies after augmenting himself with the tentacle cells. At that point, his lust to destroy Korosensei came to a point where he no longer cared about whether he lived or died in the process. His amplified sadism is notably shown when Yanagisawa laughs at Kaede Kayano's apparent demise while stating that it was funny to see her dying.

It is implied that he sought perfection from his research, focusing on Akari Yukimura and Itona, who were both implanted with tentacles.

Etymology
Shiro means "white", which evidently references the color of his robes.

History
Kotaro Yanagisawa was once a 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 to his research. He himself admitted that he lacked passion and determination. While still a university student, he met aspiring teacher Aguri Yukimura, whose family ran Yukimura Pharmaceuticals. 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 he essentially treated her very poorly.

Needing a human guinea pig to start his project, he managed to gain custody of an imprisoned assassin nicknamed the Reaper, who was sentenced to death after his capture. Yanagisawa explained the reasoning as to why he did so, 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 sway people into doing his bidding, Yanagisawa chose Aguri to closely observe him, seeing her as equally disposable. 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 Reaper in 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 wiped out 70% of the moon, giving it its permanent crescent moon fixture. Horrified that the same thing would happen to his test subject 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 witnesses through surveillance. This drove him to violently kick Aguri to the ground; the disastrous failure of the project, his guinea pig's 'insurrection', and his fiancee's change of heart had profoundly damaged Yanagisawa's pride.

Yanagisawa survived the lab explosion, albeit not without the loss of his left eye. 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 tentacles.

First Semester
Shiro makes his first appearance alongside Itona, the latter being the second transfer student to Class 3-E. After Itona claims to be 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 paralyzed Korosensei. However Shiro did not take into account the numerous anti-sensei knives the students own, and Korosensei managed to win the battle. Shiro retreats with the defeated Itona, but vowed 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 defenses, 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.

Shiro would later head up the research in the ultimate weapon to assassinate Korosensei, and is given custody of the defeated Reaper. In an undisclosed location, he elaborates in more detail about his opinion on which student has the best skills for assassination. When it was revealed that the student in question was Akari Yukimura under the alias of Kaede Kayano, he stated that he had discovered this some time ago and had known that she stole his research.

After Kayano's first attempt to assassinate Korosensei, Shiro soon confronts her for not waiting until his preparations were complete. In response to Kayano's retort, Shiro hypothesizes that her attitude was a result of a metabolic imbalance. He attempts to warn her about the risk of using tentacles without any maintenance, but ends up having to dodge an attack from Kayano's tentacles. Once she leaves, Shiro is annoyed that she would attack her own family.

Having witnessed Kayano's almost suicidal assassination attempt at the end of the second semester, Shiro appears once more, this time unmasked, accompanied by a hooded hit man before leaving. He is immediately recognized by Korosensei as Kotaro Yanagisawa.

Final Semester
In February, Shiro (now unmasked) witnesses his hooded accomplice revealed to be the faux Reaper 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 Reaper 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 Reaper penetrate the barrier preventing their target's escape. Upon confronting Korosensei, Shiro reveals the faux Reaper'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 Reaper to attack the students. Korosensei takes several hits before Kayano attacks the faux Reaper, who subsequently impales her with a tentacle on Shiro's orders. Shiro laughs 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 color black represents chaos and destruction and is pleased at this event. Using his final trump card, Shiro injects the Reaper with a stimulant to increase his power. 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 color with every emotion known, creating pure white energy. Korosensei then unleashes the energy in a powerful blast that tears apart the faux Reaper and sends Shiro flying backwards. Shiro is confident that the cells he injected himself with will protect him, until he looks up and realizes 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. He cries out in denial before passing through the shield, destroying the tentacle cells that were implanted in him, leading everyone to believe he was disintegrated to nothing.

Aftermath - Time After Time
Yanagisawa had survived the contact with the anti-tentacle barrier, but he was maimed for life and would need to be cared for by the very types of people he once looked down on. Having failed in both revenge and research, his antimatter research is deemed by world leaders to be impractical for both military and economic purposes, and thereby terminated for good. In addition, his position as an heir of his own family was revoked. While his nemesis Korosensei departed with peace and much love from Class 3-E, Yanagisawa was left alone with no one. This scene was omitted in the anime.

Even so, Kayano, his former sister-in-law, holds a sliver of hope that one day he might use his brilliance for others once he realizes that he is unable to live without the help of other people.

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 abandon Itona.

Aguri Yukimura
Aguri was Yanagisawa's fiancée before the explosion. Their relationship was an arranged one and Yanagisawa only engaged with her in hopes of her own determination rubbing off on him. As a result, 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 but puts up with him as it was the duty of a fiancee to support her husband-to-be.

Akari Yukimura
Before becoming Shiro, Kotaro acted friendly to his fiancée's 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 growing suspicious of Shiro and attempted to have Aguri end the relationship with him. He eventually discovered that Akari took a sample of 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 remained 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 due to her prodigious acting skills, Kayano was the best qualified to become an assassin.

Korosensei
At the time Korosensei was the Reaper, Shiro viewed the latter as nothing more a mere test dummy to be experimented on and constantly mocked him for his inability to do anything about it. However, upon realizing he had been manipulated by him all along on his research, Shiro went into a deep rage and his pride was subsequently shattered. He would then recruit Itona and later aid Kayano to kill his former lab subject now called "Korosensei", but both of them ultimately failed.

Shiro's lust to kill him was further amplified after injecting himself with modified tentacles, and he exploited Korosensei's desire to protect his students by commanding the second Reaper (whom he had augmented tentacles into) to attack them. However, he was later sent into the Anti-Tentacle barrier surrounding them all and as a result, he became permanently disabled.

Abilities

 * Scientific Knowledge: Yanagisawa possesses extensive knowledge in the fields of science, particularly in quantum mechanics, biochemistry and physical chemistry. His scientific prowess was used to create what would become tentacles.


 * Magic tricks: Yanagisawa is skilled at performing magic tricks. He utilizes this skill to make himself appear more charming and to hide his true personality.

Post-tentacle augmentation

 * Antimatter physiology: After implanting his vital organs with tentacle cells, Yanagisawa's body becomes almost entirely composed of tentacles. As a result, this grants him a multitude of new powers, but in turn he is rendered vulnerable to anti-sensei weapons.
 * Superhuman strength: An attack by Yanagisawa was able to propel Karasuma a fair distance away from him.
 * Superhuman speed: Although there is no exact measure of how fast he can travel, Yanagisawa can travel at a speed on par with Korosensei's.
 * Durability: Yanagisawa was able to withstand a large energy blast discharged by Korosensei.

Quotes

 * (His motto) "A genius' moment of inspiration is greater than an average joe's entire life."

Etymology
柳沢 誇太郎
 * The name Kotaro means "proud" (誇) (ko), "thick, big" (太) (ta) and "son" (郎) (ro).
 * Kotaro's surname Yanagisawa means "willow" (柳) (yanagi) and "marsh" (沢) (sawa).

Trivia

 * 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.
 * His quote in the "Tentacle Zen" page of Volume 16 is, "Tentacle cells do exist!"
 * In the live action movie, the Reaper's tentacle form is used by Shiro.