His skin color changes depending on his mood, as well being able to move at a maximum speed of Mach 20 (24,500 km/h), which allows him to travel across the world in a short amount of time and makes him a hard target to hit.
He does however have a number of weaknesses, mainly to special rubber knives and BB pellets specifically designed to work against his body.
Koro-sensei was born in the slums of an unknown third world country, where he soon learned he had a natural talent for killing and never bothered using his empathy.
However, after being captured and used as a guinea pig (although he can escape at any time, he chooses to stay to develop superpowers, subtly guiding the experiments so that he can profit from them), he begins to learn how to use compassion and empathy from a female intern with whom he becomes close.
According to Koro-sensei, his lifespan will be over by the end of the school year in March, and if he is not killed by then, the whole planet will perish with him due to the explosive release of the anti-matter in his body that was generated by the experiments performed on him.
After a vicious clash with Kōtarō Yanagisawa, the scientist responsible for his current condition, and his former apprentice, the second 'God of Death', and using all of his energy left to save Kayano who was mortally injured during the confrontation, an exhausted Koro-sensei bids farewell to his students and finally allows them to kill him, with Nagisa ultimately doing so by stabbing him in the heart.
During the graduation ceremony, Nagisa learns, much to his joy, that before his death, Koro-sensei talked with his father as well and helped him reconcile with his mother.
After, completing his studies, he becomes an intern at Gokuraka Private High School assigned to a dysfunctional class full of male delinquents and gang members, much to his chagrin.
She experiences a slight inferiority complex when it comes to sex appeal, especially her bust size, as she usually gets flustered towards girls with large breasts.
Believing Koro-sensei to be her sister's killer, she stole and implanted tentacles from Shiro's facility onto the back of her neck and changed her identity in order to infiltrate Class 3-E, using her acting skills to hide her extensive blood lust from everyone until she was ready to make her move.
However, using her tentacles in battle puts her life at risk, just like what happened with Itona, and before she dies, Koro-sensei manages to remove them from her body once Nagisa calms her down with a French kiss, shocking her to the point of passing out.
She then spends the winter break recovering at a hospital, and upon returning, decides to hide her newfound feelings for Nagisa in order to not give him trouble during their mission to assassinate Koro-sensei.
During Koro-sensei's battle with Yanagisawa and the second God of Death, Kayano attempts to intervene, making use of the kinetic vision she obtained during the time she implanted the tentacles in her body, but is mortally wounded by the enemy, and to save her life, Koro-sensei performs an intricate surgery to restore her body up to the cellular level in order to save her life, a procedure he developed to ensure that the tragedy that happened with her sister would not be repeated, but exhausting himself in the process.
He was suspended from school prior to Koro-sensei's arrival by his class' homeroom teacher due to his extremely violent behavior.
He and Nagisa have a tense friendship, which slowly develops into a heated rivalry, culminating in a battle to decide whether they should kill Koro-sensei or not.
Conventionally, it is composed of twenty eight students who could not either compete with the school's standard level of study, nor comply with its overly rigid code of discipline, among others who were subjected to some form of unfair punishment.