Katsuhira Agata is the main protagonist of his story. He is a high school boy who stands out primarily for his profound and pervasive emotional detachment. He has a tall, slim build with droopy grey eyes and is consistently seen with an expressionless face. A notable feature of his appearance is his hair, which is mostly white with traces of brown, a physical change that resulted from a traumatic childhood event.

Katsuhira's defining characteristic is his complete inability to feel physical pain. This condition also extends to his emotional life, leaving him seemingly indifferent to both joyful and distressing situations. His mood rarely fluctuates, and he shows little interest in the people around him, including the classmates who constantly bully him for money. Due to this numbness, he has trouble forming connections with others and is often perceived as oblivious to their feelings. Despite this, he is not entirely devoid of care for others. He demonstrates a willingness to risk his own safety for the well-being of his peers, such as when he jumps off a bridge to prevent someone else from doing the same.

This state of apathy is not innate. As a child, Katsuhira was a shy but energetic and caring boy. His personality was drastically altered by his involvement in the Kizuna Experiment, a project designed to connect people by sharing pain. It is revealed that during this experiment, he was permanently bonded to a girl named Noriko Sonozaki. This connection caused most of his physical pain and a significant portion of his emotions to be transferred to her. The result was a diluted personality and an indifferent attitude towards the world, while Sonozaki was left burdened by his suffering.

His role in the story is central, as he is selected to be part of a new iteration of the Kizuna System alongside six other teenagers. Through this system, the group is forced to share each other's physical and emotional pain. For Katsuhira, this process becomes a journey of rediscovery, as the shared experiences and the connections he forms slowly begin to chip away at his emotional numbness and help him regain the memories of his past.

His key relationships are fundamental to this development. His bond with Noriko Sonozaki is the most significant, as she is both the keeper of his lost pain and the key to his past. He feels an unexplained pull towards her, eventually realizing he has romantic feelings for her. His childhood friend, Chidori Takashiro, cares for him deeply and has witnessed his transformation from the boy he used to be. She calls him "Kacchon" and acts as a constant, supportive presence in his life while grappling with her own unrequited love for him. His relationship with Hajime Tenga is also complex; while Tenga initially acts as his protector, their connection is tested by jealousy and frustration.

Katsuhira's only notable ability is his lack of pain. This is technically a disability, but it gives him a unique position within the Kiznaiver group. When other members are injured, he does not experience the shared pain, allowing him to act without inhibition in situations where others might be incapacitated. However, his body is not invincible; he still receives physical wounds and bleeds just like anyone else. Throughout the narrative, he is shown to be remarkably forgiving, holding no grudges against those who have hurt him, including his bullies and even his friends who lash out in frustration. By the end of his journey, he gradually returns to a more expressive version of his former self, ultimately regaining the ability to feel the full spectrum of human emotion and pain.