Katsumi Yagira

Katsumi Yagira is a student in Korosensei's Class 3-E of Kunugigaoka Junior High School and the main deuteragonist of the series.

Prior to being a student, she was the assassin known as "Certain Death".

Personality
Katsumi can be described as very kind, but can also beat one up with a smile still on her face. She has a huge appetite and is shown to eat many times. She acts respectfully towards those who are kind to her yet when speaking about them, she can be embarrassed about it. However, anyone who makes her angry will receive a fitting punishment from her.

Growing up as an orphaned child assassin who was treated as a mere tool, Katsumi was not educated like a regular human being and did not understand human feelings well. Even before she was found and taken in as an assassin, she couldn't speak or understand speech, only understanding orders to kill. She did not possess any sense of right or wrong from the start; she didn't understand what could be considered righteous or erroneous. She merely chased after the adults who gave her orders. She followed the idea of "kill or be killed," thus she could kill countless people without feeling guilt or remorse. She did anything that was asked of her and always followed orders, which was a trait from her life as an assassin. Even after she betrayed her master with Korosensei, Katsumi continued to think of herself as a "tool" and believed that it would be appropriate to discard her if she can no longer be used as a weapon.

However, the one who showed her the life outside of being an assassin was Korosensei. He was the one who taught Katsumi speech, writing and many other things which made her into a slightly more expressive person. Katsumi is seen to be a persistent, dedicated, and dignified girl, and she is described as someone who is likely to not back away no matter how much others urge her. Korosensei and Katsumi had a special bond; she treasures Korosensei very much, which made her want to learn what his words, "I love you," truly meant. In doing so, Katsumi made her own decision and followed her own path for the first time in her life.

Despite being on the calm and accepting side, she still knows when to draw a line, and can be incredibly punishing when someone, especially Korosensei, does something inappropriate, such as spying on her while she is bathing. She has a scary side of her is mostly shown on Korosensei and Karma with a smile still on her face of which may appear to them as a "monster."

It seems that Katsumi is bored with everything, as there is no one who can barely stand a chance against her. In spite of that, she was implied to have been a bit hopeful of facing eventual defeat one day by someone stronger than her through combat. At the same time, she hardly ever bears emotion or sadness for the death of someone from the same side.

After growing to understand human emotions, Katsumi began feeling guilty about having killed so many people as an assassin. Katsumi also began to understand the words Korosensei told her: that she is on fire and has many burn marks, words that referred to the actions she took as an assassin. This made her become more apologetic to people close to her, as well as more aware of people's suffering, in which she turns into a much more empathizing and helpful person who does her best to help other people and her classmates no matter the difficulties. She feels guilty for people's pain and suffering even though she wasn't the one who caused it.

She also came to value the human life a lot, and she made a pledge to never kill anyone else again since she knows that anyone she kills will never be able to convey their feelings to their families and loved ones again. She never wants to go back to being a tool and deems that she doesn't need anyone's orders to live her life anymore, which was a wish that Korosensei had deeply wished for. Towards the end of the series, Katsumi comes to understand the phrase "I love you", as well as her own and others emotions and feelings much better, to which she becomes a significantly expressive and happier person.

Abilities
Katsumi is highly capable and skilled. As a former assassin who was trained from a young age, Katsumi excelled in combat, and her fighting abilities and strength on the battlefield made her seem more like a "weapon" than a human being. She had no problem killing countless others if given the order to do so. Due to her strength and actions taken during her time as an assassin, Katsumi came to infamously be known as "Certain Death."
 * Fighting Skills and Strength - Katsumi's combat capabilities are monstrous and described to be far beyond what a human is capable of. Even as a child, she was capable of massacring numerous adults (soldiers, prisoners) with ease. Her combat knowledge and skills were from before Korosensei found her, as she already displayed her prowess when she first met him.
 * Katsumi, as seen when she fought as an assassin, has exceptionally high stamina and speed. She is agile, has fast movements and can easily dodge other peoples attacks while attacking others. She doesn't react very strongly to pain and can endure her pain while attacking others and helping her comrades. She is strong to the point where her skills are even more impressive than Korosensei, who had been trained in battle and martial techniques as well as wielding weaponry as an assassin. She can defeat and/or kill several adults one after the other in a short period of time.