Katsura is a young man, likely in his late teens. His hair is black and hangs in front of his eyes. He wears a black turtleneck shirt, dark grey pants, black gloves, and dark shoes. Over his clothes, he wears a black coat with a hood. In the past, he appears to have worn a gakuran school uniform.
Katsura appears to be wholly focused on what Kunikida told him two years ago, to the point of almost being obsessed. He is intelligent, having planned ahead with regard to the bombs and their setup. His desire to destroy Kunikida is not driven by revenge, but his desire to get rid of Kunikida's ideal, as well as test it. He seems to consider himself weak, and much of his actions seem to indicate an inability to accept himself.
When Kunikida chooses to blow himself up with Aya, Katsura appears to look upset as his "testing" of Kunikida's ideals did not work out the way he had hoped. Kunikida tells him again, that he won't let anyone die in front of him, and when Katsura is being arrested, he smiles, perhaps accepting Kunikida's ideals and beliefs.
Later when speaking to Fyodor, Katsura seems to have changed, having great respect for Kunikida, subtly defending him and saying that Kunikida's ideals are the real thing.
Two years ago, he was arrested by Kunikida after planning to detonate an explosive in his school. Kunikida spoke to him, telling him to "live on with courage" and those ideals stuck with Katsura. Katsura found those words condescending and planned for two years to beat Kunikida in order to erase the words Kunikida had said.
Katsura delivers a suitcase with an explosive in it to Aya before running away. When Kunikida and Aya go to search for him, they notice that bombs have been placed on the train tracks. Taking advantage of a distracted Kunikida, Katsura manages to strap a bomb to Aya. He uses this as a way to disarm and tie up Kunikida. Katsura beats Kunikida with a stick, angry about Kunikida's ideals that have haunted him. Kunikida manages to trick Katsura into opening his notebook, and releases a stun grenade. Using the distraction, he breaks free and ties up Katsura.
Despite being tied up, Katsura presents Kunikida with an ultimatum: choose between disarming the bombs placed on the train tracks, or disarm the bomb attached to Aya. After Kunikida chooses to blow himself up with Aya, Katsura is shocked. When Kunikida and Aya are revealed to be safe through Yosano's ability, Katsura is taken into custody by the police. 
During the events of cannibalism, it is revealed that Fyodor has kidnapped Katsura. He yells that Kunikida's ideals are the real thing and that Kunikida's spirit is not easily broken. His whereabouts are currently unknown. 
- (to Doppo Kunikida) "Kill the girl to prove it! That your 'Ideal' is just an idiotic dream! That you aren't any different from someone like me!" 
- Katsura is named after the main character of Kunikida Doppo's work "An Uncommon Common Man."