Katsuyuki Tohma is an athletic and popular upperclassman, an accomplished sprinter known for his good looks. He becomes the focal point of a romantic conflict between identical twins Ruka and Rumi Kobayakawa, both of whom develop feelings for him. Katsuyuki uniquely possesses the ability to visually tell the twins apart despite their identical appearance.

He initiates the central conflict by confessing his feelings specifically to Ruka and rejecting Rumi. This occurs before the twins encounter a mysterious bacterium in a cave mass grave, an event that kills their fellow track team members. Post-exposure, Rumi undergoes a drastic personality shift and gains dangerous psychic abilities. Katsuyuki remains devoted to Ruka despite Rumi's escalating threats and aggression.

An early kiss with Ruka transfers antibodies from her strain of the bacterium to him, granting him immunity. This immunity protects him from Rumi's attempts at control or infection through her strain, positioning Katsuyuki as Ruka's protector. Throughout the conflict, he actively supports Ruka as they evade Rumi's violent pursuits. His immunity serves as a major obstacle to Rumi's plans, preventing his manipulation or control.

Katsuyuki has experienced family loss, indicated by rainfall during his family's funeral. His role remains consistent as Ruka's primary supporter and protector against Rumi's aggression, with no significant change in his personality or objectives throughout the narrative.