Katsuhira Agata

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 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 for 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 for this sight, Noriko says their relationship will start off pure unless Katsuhira supposedly pushes her to the ground one day.

Appearance
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.

Chidori Takashiro
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 for cleaning his house with him and making him lunch, being concerned about his bullying issues even to the point of getting upset when he said it didn't matter, and initially getting jealous when he chooses to talk to Sonozaki.

Yoshiharu Hisomu
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.

Hajime Tenga
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 was 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. But in episode 11, Nico shows that her bond with Katsuhira has developed even deeper and said that she still want to be friend with Katsuhira and share their problem together while hopping around.

Quotes
"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 disdain 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.

Trivia

 * 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.