It was there that he met and befriended Aguri Yukimura, a teacher who was at the time in an arranged and abusive marriage to Yanagisawa. She was brought in to keep a closer eye on him as Yanagisawa considered her easily expendable if he attempted to break free and take her hostage. As the tests continued, the Reaper began to slowly bond with her and became fond of her as she worked on compiling exam questions for her class, and it was during this time he learned what compassion was. The students' actions had also attracted the attention of the professional hit man known as the The Reaper. When Korosensei was away, the hit man took the class hostage to lure the target in and Korosensei was soon captured. They were later freed when Karasuma defeated the Reaper.

During the semester, Korosensei had been challenged to a duel against the second transfer student Itona Horibe, accompanied by the mysterious Shiro. To his and everyone's surprise, Horibe has the same tentacle implants as Korosensei which explains why Horibe had earlier called Korosensei his 'brother'. Although visually disturbed by Itona's tentacles and the well planned strategies devised by Shiro, Korosensei outmaneuvers the pair and defeats Itona. Before Korosensei could get any answers regarding Itona's tentacles the pair retreated but not before vowing to return. The pair kept their vow and returned to ambush Korosensei but he was rescued by the students who were none too pleased that they were used and almost killed indirectly by Shiro. 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.Later that night, 10 of his students collapse to an artificial virus. The mastermind behind the virus puts them to ransom and demands that Korosensei be delivered by Nagisa and Kayano to the mountain hotel in exchange for the antidote. Although unable to move, Korosensei assists the 15 remaining students to infiltrate the hotel and even helped call out commands while simultaneously distracting one of the hired hitmen. When the class discovers the mastermind as their short-lived PE teacher Akira Takaoka, Korosensei is forced to watch him and Nagisa battle it out for the antidote. The situation was so grave that Korosensei grew worried and even advised Karasuma to intervene should anything bad happens. Fortunately, Nagisa defeated Takaoka, and the ill students had never been in any danger; the virus was a fake. The next day, Korosensei was sealed away in concrete slabs by Karasuma in a vain effort to kill him, but he manages to return to his normal form.

Korosensei later watches the battle unfold from a raised platform and commentates on some of the individual students' hidden talents. When the fight is reduced to the last man standing between Nagisa and Karma, he compares each of the boys' specialized skills. Eventually, Nagisa wins when Karma surrenders voluntarily, but the class has been reunited under a common goal once more. Karasuma later speculates that the war is what Korosensei had planned all along, another lesson to educate the students. Within the UK and USA the price you see is the price you pay - you won't be charged any extra fees. The world will always be filled with injustice. If you have time to give up or hold a grudge against that injustice, then use that time to enjoy battling those injustices instead." 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.

His ability to regenerate his tentacles, healing others, bestowing wisdom, and appearance is reminiscent of the Akkorokamui. Class 3-E students: Arguably Korosensei's greatest weakness. Although students of 3-E class are subtly aware of this, only three students ever made actual attempts to exploit this factor: Karma Akabane, Nagisa Shiota (using himself as bait), and Ryoma Terasaka (using Nagisa as a suicide bomber the first time, second when he was unknowingly manipulated by Shiro into nearly drowning his classmates, but has redeemed himself since). Exploiting this weakness means one has no qualms of discarding lives of his/her classmates, and even their own; Yanagisawa has demonstrated these qualities. 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. Their shared desire to teach ultimately results in a lot of trust between them. In the final volume, Asano is described as Korosensei's "greatest supporter," and when Korosensei reflects on which assassination attempts were the closest to succeeding, he mentions that he wasn't too worried about participating in Asano's game because he knew the explosion wouldn't be so strong as to injure the other students.

Honestly, there isn't much meaning to the splendid names given to you by your parents. What does have meaning, is what the person behind that name does during their actual lifetime. The name doesn't make the person. The name simply remains gently within the footprint left on the path a person walks." 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 the Class E students and professional assassins, showing to be extremely observant and stealthy at planning and execution.To Class 3-E) "As you go through life, the mighty current of society is bound to get in your way, and there will certainly be times things don't go as you'd hoped. When this happens, do not look to society for a cause. Do not renounce society. Frankly, you'd be wasting your time. Instead, just say, "That's life!" and muddle your way through with frustration." 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 melt 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. 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. His greatest feats appear to be coming up with inventions leaps and bounds above contemporary science in a matter of days. Poison Immunity: Korosensei is immune to toxic substances that would definitely kill an ordinary human. Although, such substances do affect him, such as by transforming his head.

On the final night of the deadline, Korosensei and the class happily reunited. Briefed of the situation and that the satellite weapon would fire again in 90 minutes, he imparts one last life lesson to his students - to confront and navigate through society head-on and not reject it as he once did. Rio Nakamura presents their teacher with a small cake to mark the event Aguri declared to be his birthday (the day she and Korosensei first met each other). Having not eaten any sweets for nearly a week, Korosensei starts drooling on the cake but nonetheless feels happiness. However, before he could blow out the candle, the cake was destroyed by Shiro, accompanied by Korosensei's former apprentice (No. 2), who is now a hideous tentacled monster. Much like the first semester Korosensei continued teaching Class 3-E some valuable life lessons. He gave the class some practical experience to their newly-learned ability to free-run taught by Karasuma by organizing a game of Cops and Robbers, and inspired Yūma Isogai to lead his class to victory against Class A during the Athletic Festival. Even though he taught him everything he knew, all while still instilling respect and fear in him and letting him know the difference in power between them, the first Reaper still saw him as his tool rather than a real student, which eventually cost him his apprentice to and abandoned him at a prison after realizing his teacher's view of him. Korosensei loved sweets almost to extreme cases, [5] 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.He and the class meet her at Kunigigaoka Park, and the two start to fight. Immediately Korosensei senses Kayano's subconscious fading as her tentacles start to dominate, and asks the class to find a way to calm Kayano's bloodlust before she dies. Korosensei intentionally reveals his main weak point - his heart behind his crescent moon tie - and Kayano strikes. Although being gravely wounded, Korosensei pins her down stating his promise to her sister not to let any of his students fall, allowing Nagisa to approach Kayano and kiss her into unconsciousness. Korosensei removes her tentacles and sees Shiro (who Korosensei now recognizes to be his jailer Kotaro Yanagisawa) watching the whole thing with a masked accomplice. When Kayano woke up filled with regret, Korosensei tells the class about his past as the original Reaper, bonding with Aguri during his captivity and promising to teach her students before she died during his escape. He also stated that even if no one managed to assassinate him by March, he would die along with the entire world. Teaching Ability: With his immense intelligence, he was able to single-handedly teach multiple academic 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, all of which seemingly being done simultaneously (including providing individual teaching to every student of Class 3-E simultaneously, focusing on the subject each of them needs to study most and even changing his headband as he goes).