Koro-Sensei

Koro-Sensei is the homeroom teacher of Class 3-E of Kunugigaoka Junior High School, and the central character and main antihero of Assassination Classroom anime and manga.

He claimed to be responsible for creating the permanent crescent moon, and added that he planned to destroy the earth after teaching Class 3-E for a year.

Prior to being a teacher, he was the assassin known as "The Reaper".

Appearance
Korosensei has the appearance of a yellow bipedal octopus. He is about three meters tall and possessed a large, bulbous head with a large smile that rarely breaks or moves and small, beady eyes. He uses two tentacles as arms, each having two feelers as fingers, and utilized the rest of his tentacles to walk. Korosensei has a pair of hidden nostrils, which make him appear to have four eyes if detected. Due to his constant grinning, he has the ability to change his color depending on his emotions.

Korosensei wears a black academic dress, a small black squarish academic cap with a yellow tassel, and a large black tie with a yellow crescent-shaped moon on it, which is a gift from Aguri.

As a human, he had pale skin, a slender build, dark eyes, and short black hair that reached to his nape. He sports an amicable smile, which was initially used to manipulate others. When carrying out assassinations, he donned formal clothing. After being taken prisoner by Yanagisawa, he was put in patients' scrubs until his escape and transformation.

Personality
It was revealed that he was raised in an environment "where everybody could easily betray one another to the point that one could only trust oneself". Through these hardships, he decided to become an assassin in order to survive. Despite gaining an apprentice, he only saw him as a tool for his goals, which drove the latter to finally betray his master.

However, after meeting Aguri Yukimura, he finally knew what compassion really was--becoming much more kind and compassionate. According to Aguri, if he was born in a peaceful country, his personality would be "someone a little perverted, with a good head but a few screws loose, petty about small things, and also very stubborn." He has/would have "a gentle smile not just for business, but because he is a gentle person himself." This prediction happens to reflect his personality; which became forefront after he answered the tentacles.

When first introduced, Korosensei was a jovial individual who demanded absurd and seemingly mundane requests despite claiming he would be the cause of the planet's destruction in a year. He was also rather kind to others in general and could switch from silly and childish to serious on the fly. According to Nagisa, Korosensei also was unintentionally petty at others at times and loved gossip, both traits heavily noted throughout the series.

From time to time pieces of his old personality as The Reaper were visible such as when Terasaka tried to sacrifice Nagisa to kill Korosensei and Korosensei menacingly threatened all of their families if they tried another self-harming attempt like that. In that moment he resorted to his old method of teaching through fear. As the series progresses his character development renders him more and more capable of true empathy and learning to use that empathy to teach lessons instead of through fear and reward like he had with his first student.

Korosensei deeply cared for his students' emotional and physical well-being and would lose his temper whenever any of them were put in danger, even by one another. On the annual school trip to Kyoto, a few of his students were assaulted/kidnapped by a gang of high schoolers hidden in an alley. After searching the entire area painstakingly, he infiltrated the abandoned building where two kidnapped students were being held at and single-handedly beat up all of the high schoolers, scaring them away.

One of Korosensei's attributes was the fact that he was able to connect to each student individually, being an objective, empathic, and observant person who will do almost anything to help his students. This was the reason why all the students respect him despite the fact that he was going to blow up the Earth in a year. Taking care of each student's needs, Korosensei would sometimes be piqued by a student's current problems or interests and would try his best to guide the student along or solve it for them.

Korosensei loved sweets almost to extreme cases, and was also shown to be a very perverted character, a trait he shares with his student Okajima. This characteristic was revealed to be the side effect of the experimentation which made him unable to hide his true feelings.

Powers

 * Antimatter physiology: Due to the human experiments of producing antimatter in a living body by Shiro, Korosensei's entire appearance is an octopus-like creature with multiple tendrils that function as his tentacles. Anything aside from Anti-Sensei Weapons will be ineffective in harming him. It is initially believed that he would self-destruct by the time of Class E's graduation; however, it has since been proven that the chances are minuscule at best.
 * Protective coloring: Korosensei can change his skin into any color, utilized for expressing emotions as well as disguise and camouflage.
 * Mucus: The standard trait of an antimatter organism, the mucus that Korosensei has varied uses for support purposes, such as waterproofing, cleansing, adhesive and the replacement of tears.
 * Liquefaction: After he drank the 4th poison made by Manami Okuda, Korosensei developed an ability to control his fluidity. He can morph into a mass of mercury-like slime. In cost of his massive body and tentacles, Korosensei can fit through any space while retaining most of his abilities.
 * Absolute Defense: Another trump card never used until the Okinawa trip, Korosensei can shrink his body into a sphere and the remaining energy from the transformation hardens as a crystallized second layer over his spherical body, making him impervious to harm. After twenty four hours, the external shell liquefies and he will slowly absorb energy over a period of time until he fully regenerates. The only drawback to this in the span of a day, he is rendered immobile and is susceptible to being carried around. The only mechanism that could potentially break through this form is the Spear of Heaven.
 * Energy laser: An ability developed from the Absolute Defense. Korosensei can channel a portion of his inner energy to his tentacles in order to discharge a laser blast.
 * Molting: A once-a-month occurrence, Korosensei can shed his old skin and use it as a protective membrane either on himself or another individual, protecting them from being hurt even from military-grade explosives. Afterwards, his reaction speed may temporarily be reduced, but he can still be fast enough to dodge attacks from the class.
 * Regeneration: If one or more of his tentacles are maimed, Korosensei can regenerate the severed limb. However, this weakens his stamina.
 * Surgery: After he was unable to save Aguri from her fatal injury, he began learning about surgery methods, which he first demonstrated on Azusa to remove her brain tumor without damaging her. He later uses his knowledge to heal and revive Kayano from receiving the same injury as Aguri.


 * Superhuman speed: Korosensei can travel up to the speed of Mach 20. His normally soft and jelly-like head hardens under immense pressure as a process known as the dilatancy phenomenon, which presumably lowers his wind resistance, allowing him to reach inhuman speeds. The only known objects to surpass this are the reusable space shuttle and the transformed Reaper.
 * Superhuman reflexes: Korosensei can react and dodge multiple fires of Anti-Sensei bullets and move away from any harm. His reflexes also allow him to react fast enough to catch or deflect any flying projectile in mid-air using what he has on hand.
 * Speed replication: Korosensei's amazing speed allows him to seemingly copy himself dozens of times using after-imagery. Not only can each individual after-image maintain a task for extended periods of time, but Korosensei's "speed copies" are all capable of taking on widely-different tasks both individually and simultaneously without confusing themselves. This technique was primarily used by Korosensei to teach all of Class 3-E one-on-one when a difficult test approached, allowing Korosensei to work with each student separately to work on their problem subjects.
 * Superhuman senses: Korosensei's sensory perceptions are paramount, and can detect even the most minuscule of changes in temperature, light, sound, and smell. Despite having no visible ears or nose, Korosensei has proven to be able to hear things no normal human can pick up on, as well as detect the most minor of aromas, which aided him greatly in evading the assassination attempts of Class 3-E. Korosensei is especially talented in identifying people by their smell, as well as certain materials (such as explosives and poisons). His eyes are also quite keen, and can keep track of objects going thousands of miles per hour without difficulty, as well as detect the slightest of movements that occur quicker than the blink of an eye.
 * Superhuman digestion: Korosensei can digest anything aside Anti-Sensei weapons, as his teeth can crush any hard materials. Whether he eats something or it enters his body, he can either molten the food first then eject it, or it will just be digested. Although it is not guaranteed that Korosensei will receive no irregularities from what he has eaten, it usually has no tactical disadvantages if it is not a weakness for him.

Abilities

 * Genius Intellect: Even before Korosensei gained his powers, he possesses a gifted mind which allows him to learn and understand complex knowledge in a matter of minutes. Since gaining his speed, Korosensei has become hypercognitive, and can process information thousands of times faster than the average human being. With his immense intelligence, he was able to single-handedly teach all the subjects and produce impressive results from Class 3-E. His incredible mentality combined with his speed also allow him to multitask on up to dozens of jobs at a time (teaching the entirety of Class 3-E individually and then some), all of which seemingly being done simultaneously. His greatest feats appear to be coming up with inventions leaps and bounds above contemporary science in a matter of days, such as Ritsu's upgrades and the chemical that granted him Liquefaction.
 * Assassination: As the original Reaper, Korosensei was considered the world's best assassin prior to being betrayed by his disciple. As a result, Korosensei can combat nearly every assassination attempt utilized by both Class E and professional assassins, showing to be extremely observant and stealthy at planning and execution.
 * Eidetic Memory: Korosensei is able to store all that he has learned and use them to his advantage, which led to him being the strongest assassin in the world prior to his capture. In order to become a competent educator to Class 3-E, he has memorized all of the text books in Japan so as to be able to coach his students where their weaknesses lie.
 * Genius Chemist: He was able to create a chemical granted him his Liquefaction ability.
 * Mechanics: Korosensei was able to upgrade Ritsu, designing ad implementing approximately 985 modifications and upgrades within less than a single day. He also upgraded her AI to grant her some form of autonomy and sentience.

Trivia

 * Korosensei has never actually known his real name.
 * Korosensei is usually represented by a octopus due to his appearance of his head and tentacles. Aware of this, he often draws octopus mascots when scoring exam marks and makes jokes about octopuses.
 * He also has a nature similar to an octopus, as he is highly intelligent for his survival skills.
 * His ability to regenerate his tentacles, healing others, bestowing wisdom, and appearance is reminiscent of the akkorokamui.
 * In the popularity poll in Volume 12, Korosensei was ranked 3rd.
 * Korosensei is ambidextrous, being able to write/perform a task with either hands, and being able to work with both hands at the same time.
 * According to Aguri, Korosensei is not Japanese.
 * His necktie is a present from Aguri Yukimura--though, the crescent moon motif is his own design.
 * He once took on the guise of a university professor.
 * Korosensei is portrayed by Jun Fukuyama, who previously portrayed another manipulative protagonist, Lelouch vi Britannia, from the Code Geass series.