- "Are you God? I'm just a puny insect. I cast out my parents, left my child, dismissed their love and led a selfish life. Just like an insect. No, my existence is even lower than an insect's, but that's how I've lived and this is my punishment."
- —Katsuro to Josef Heiter[src]
Not much is known about Katsuro's background, except that he treated his family poorly. He threw his parents out his life and had a child who he eventually abandoned.
A Japanese tourist in Germany, not able to speak a word of English nor German, Katsuro is sedated by retired surgeon, Josef Heiter, who acquires him after one of his previous victims was found to be incompatible with fellow captives Jenny and Lindsay. Because of the language barrier, Katsuro is unable understand what is going one when Heiter explains the surgery he will preform on them while Katsuro is tied to a bed alongside Lindsay and Jenny, instead he demands to be released in Japanese and calls Heiter a Nazi. Heiter then sedates Katsuro who begs him not to. When Lindsay tried to escape while dragging Jenny who's unconscious, she apologises to a sleeping Katsuro for not being able to rescue him. Her plan, however, fails and Dr. Heiter moves on with the surgery, connecting the three captives mouth-to-anus to form a "human centipede". Katsuro is placed in the front with Lindsay and Jenny sitting behind.
As the centipede, Katsuro is treated like a pet, trained to fetch Heiter a news paper and given food in a dog bowl. During dinner, Katsuro bites the doctor in the leg in retribution but only makes it worse for himself as the Heiter kicks him in the face and holds him up by the nose, threatening to pull out his teeth if he does it again and calling him a "kamikaze shithole". The next day in his backyard, Heiter mocks Katsuro and prompts him to bite on his boots. He crawls away, turning his back as this ticks off Heiter. In this event, Katsuro suddenly needs to defecate and does in Lindsay's mouth, praying for forgiveness while Heiter demands that he "feed her". He is shown trying to hold it back, but is failing while he defecates, causing Lindsay to gag.
Katsuro eventually irritates Heiter so much by screaming constantly, rendering him unable to sleep at night. As Heiter goes to confront Detective Franz and Voller while checking the health vitals of the centipede, Katsuro sees an opportunity to escape by equipping himself with Heiter's scalpel. As Heiter returns to the room, Katsuro attacks him with the scalpel, stabbing him in the foot, calf as well as biting off a piece of skin from his neck, leaving him to bleed to death. Katsuro then takes charge and leads the girls out, ferociosly climbing up the stairs and to Heiter's room where Lindsay leads him to the glass wall they hope to escape through. She further directs him to a lamp to smash it open, but just before he can, Heiter arrives, on his stomach, armed with his scalpel. Katsuro takes a shard of glass and confronts the doctor. Nothing happens, however, Katsuro withdraws from hostility and goes to a calm place where he believes Heiter is God and this is his punishment for mistreating his family. Feeling lesser than an insect, he hopes he can at least die with dignity as a human being, so he slices his throat with the glass shard and dies, leaving the two girls incapacitated.