Kei Munakata's Abnormality awoke at a young age; while he was walking through a city, he realized how fragile life is and experienced an overwhelming impulse to kill. He does not actually want to kill others, however, and so isolated himself afterwards, putting on a fake persona of viciousness and even creating a false file on Japan's Most Wanted list. He joined the Flask Plan and the Thirteen Party in order to try and find someone who would be friends with and trust him, despite his killing impulse. While there, he was taught the use of the Hidden Weapons skill by Maguro Kurokami.
A few years later, Medaka Kurokami and her Student Council discover the Flask Plan and the ambitions of the school's chairman. They break into the Flask Plan's laboratory to stop it, and come across Kei on the second floor. He strikes by surprise, coming closer to victory over Medaka than any other antagonist in the story, bar the final. Zenkichi steps up to challenge him, and they fight. Zenkichi realizes that he doesn't actually want to kill people, and uses this to win, after which the two become friends. He later sides with Zenkichi against the Plus Six, and fights evenly with the Plus Six as part of the Loser Team. He, like the others, is quickly overpowered by Misogi Kumagawa.
After this, he starts training and regularly hanging out with Zenkichi, focusing on growing stronger to protect his friend. He grows skilled enough that, when he rematches Kumagawa, he not only fights on even ground but wins, using his newfound skills, his Abnormality, and his cunning. After killing Kumagawa, he loses his abnormality. He next appears when Iihiko goes on a rampage, stabbing him repeatedly. Ten years later, he has taken over the Munakata family and makes a living by hunting demons.
Battle vs. Shirou Emiya (by Leolab)