He resembles the sin of Sloth. In modern Japan, his sin is "Gudon" (The Imbecile) meaning Idiocy. His identification number that appears when the Kiznaivers experience pain is D-1.
Katsuhira is a tall, slim boy with droopy, grey eyes. He often has an emotionless expression on his face. He had brown hair as a child, however, the majority of his hair turned white in the aftermath of the Kizuna Experiment. He is usually seen wearing his school uniform which consists of a white shirt with black accents on the cuffs and collar as well as a blue tie with a black cross. He appears to be around 176 cm.
Katsuhira is rather indifferent in all situations, whether he feels pain or joy. He is shown to be emotionless and is also shown to be oblivious to the feelings of other people, despite wanting to build bonds. Due to his lack of interest in himself and others around him, he has trouble forming connections, with the exception of Chidori.
However, it seems like his lack of interest started in grade school, as Chidori's love confession stated that she wishes he would go back to his fun and caring self.
Katsuhira, despite having little emotion, does care for those around him, even to the point where he is willing to risk his own safety for the other Kiznaivers, including jumping off a bridge early in the series to prevent Tenga from doing the same thing. His mood barely fluctuates, and he pays little attention to other people. He is constantly bullied, but doesn't feel the pain.
In episode 10, Katsuhira is shown to be a shy yet energetic boy before the first Kizuna experiment. In that same episode, it is also revealed that once he was permanently connected to Sonozaki, most of his pain and emotions went to her, thus leaving his personality diluted, and his attitude towards others indifferent.
However, throughout the series, he gradually returns back to the same kind, thoughtful and sociable boy he was back then. He is also very forgiving, as seen when he holds no grudges towards others despite what they put him through, especially Sonozaki and Tenga as well as his bullies.
While technically a disability, Katsuhira does not feel pain. This means when the other Kiznaivers are affected by someone else's or their own pain, Katsuhira is not inhibited at all. Despite this, he still bleeds and takes damage when his own body is injured. It is later shown that most of his pains goes to Sonozaki due to their permanent Kizna bond.
Though Katsuhira claimed that he did not know Sonozaki, the two of them were experimented on as children, though he does not remember this. In the beginning, Katsuhira's heart starts to race when Sonozaki gets really close to him. She takes note of this and asks him why, and he responds that he has never been that close to a person unless they were beating him up. Rather than calling her Sonozaki like the rest of the Kiznaivers, he calls her Nori-chan, causing Chidori to get jealous. He is also the first to ask for her contact information and invited her to the summer training camp in episode 5. In episode 6, Katsuhira finds Sonozaki and expresses his disappointment towards her, which is the most candid thing he has ever said to anyone. As well as him being extremely truthful towards her, it seems as if he thinks very highly of her, which is why he was disappointed with her actions. It seems that she is the key for remembering his past.
In episode 8, the Kiznaivers had another mission/experiment about love. A chart is drawn with all the love interests and it is said by Noriko that Katsuhira's love interest is her. Noriko leaves the lab to see if she can draw out Katsuhira's romantic feelings. As he has flashbacks about his childhood and Noriko, he runs outside just when Chidori was about to confess to him. He saves Noriko from a falling statue and sees that Noriko herself is the girl who he had constant flashbacks about. At this point, Noriko says that he exists inside her because she wants to find herself (because she isn't capable of feeling anything) in him and possibly because it is a way to draw Katsuhira's feelings.
It is said that Katsuhira has romantic feelings for Sonozaki, but Katsuhira himself says that he's not sure about that and that he only thinks about her most of the time. It is later revealed that Sonozaki and Katsuhira were both Kiznaivers when they were younger and are still bound together. In episode 9, Katsuhira is seen voluntarily hugging Sonozaki and stating that when he hugs her, all kinds of feelings swell up inside him. Sonozaki is seen questioning this.
At the end of the series, it is noted that Katsuhira is in fact in love with Noriko and regained back all of his pain and emotions after Noriko had released them. They are both seen in a hospital where they talk and reconcile, and almost kiss before Chidori and the others interrupt them. In response to Chidori freaking out from this sight, Noriko says their relationship will start off pure unless Katsuhira supposedly pushes her to the ground one day.
Chidori and Katsuhira have been friends for a long time, even before Katsuhira became indifferent to life. Chidori calls him "Kacchon", while Katsuhira calls her "Chidori" without any honorifics, showing that he at least feels somewhat comfortable with her. She is shown to care a great deal for him by cleaning his house with him and making him lunch, being concerned about his bullying issues even to the point of getting upset when he says it doesn't matter, and getting jealous when he chooses to talk to Sonozaki.
Prior to his personality change, Chidori confesses that she was in love with him, and it gives hints that she still loves him because she blushes around him and gets jealous of Sonozaki. During his introduction. when he states that he feels indifferent towards people and doesn't care about himself, Chidori gets extremely irritated and points out how he used to be funny and nice and is driven to the point of tears as she explains how she's been trying to get him back to the way he was previously.
Katsuhira shows a fairly decent amount of emotion after seeing her in her state and revises his introduction, saying how he is happy that Chidori was in love with him, despite it being past tense. It seems that he is unaware of Chidori's current feelings for him, or simply indifferent to it. He greatly appreciates Chidori however not romantically.
When Hisomu is first mentioned in episode 3, Katsuhira states how he feels sorry for Hisomu and starts feeling the want of having a connection with another person. Katsuhira soon realizes while the Kiznaivers are chasing Hisomu that he probably won't talk to them unless only one person like himself went to him. Katsuhira goes to meet with Hisomu, who asked if he could die for him. Later in that scene, Katsuhira causes Hisomu to bliss out after jumping off a bridge into oncoming traffic..
Later in episode 5, it is shown that Hisomu was trying to hurt Katsuhira purposely, after asking him if he could hold the firewood he was chopping or if he could sit underneath it. Hisomu also states that he is especially happy to meet Katsuhira, hinting that he has a certain interest in Katsuhira.
Tenga is the one who saves Katsuhira from bullies during the first time they meet. He is shown to care about Katsuhira when he follows him home and decides to stay in his house with him, vowing to protect him. In order to prevent Tenga from jumping off a bridge later, Katsuhira jumps in his place, even though his body would still be affected from the fall. He and Chidori are the only Kiznaivers to run to his aid, while the others go to check on or prevent Yoshiharu from escaping. However, in episode 9, Tenga is seen physically beating up Katsuhira in front of the other Kiznaivers out of frustration of not being Chidori's love interest. Katsuhira holds no grudge against him and forgives him despite what Tenga put him through.
These two seem to have a budding friendship. In episode 4, Nico gives Katsuhira the green peas she doesn't like. In episode 5, she protests to the fact of Katsuhira getting hurt by Hisomu.
"I've had a high tolerance for pain ever since I was a kid." -Katsuhira explaining his disability to Tenga.
"I can't take any interest at all in what other people think, to be honest." -Katsuhira giving his introduction to the Kiznaivers.
"I'm disappointed in you, Nori-chan." -Katsuhira scolding Sonozaki after Maki Honoka was harassed by a TV crew.
"I'm not scared of high places. If I fall from a place that high and it still doesn't hurt...that would be scary." -Katsuhira as a child, speaking to Sonozaki.
- He has the nickname of "Kacchon" from Chidori.
- Unlike the other teenage Kiznaivers, Katsuhira has been part of the Kiznaiver experiment once before.
- It is shown during a flashback that as a child, Katsuhira was afraid of his inability to feel pain.
- As shown in episode 1 when the Kizna system activates within him, and implied in episode 10, Katsuhira has not completely lost his ability to feel pain.
- Katsuhira's hair colour was originally brown, but was altered due to his Kizna connection to Sonozaki.