Tomoya Mizuhara is a 14-year-old boy whose family life changes when his father remarries, giving him a 17-year-old stepsister, Ichika Mizuhara. This new family structure forms the core of his background. He is reserved, well-mannered, and often shy or timid. Tomoya reacts to Ichika with annoyance and discomfort due to her persistent attempts to enter his personal space, especially his room, and her overtly affectionate behavior. Despite his initial resistance, he eventually becomes captivated by Ichika and her friends, indicating a subtle shift in his attitude.

Tomoya harbors a romantic interest in his classmate, Mina Fujisaki. He perceives Mina as cute and sweet and treats her kindly even when she yells at him, unaware of her true narcissistic and mean personality contrasting her outward demeanor. His social circle includes classmate Mitsuru Hanazono, who often has a mind "in the gutter" but fiercely defends Tomoya against harassment, and Kouki Hayasaka, an uptight student admired for his academic abilities who is later revealed to be the brother of Ichika’s friend Ruri.

Tomoya's relationship with Ichika drives many interactions. While he frequently rejects her advances due to shyness, he shows moments of jealousy when she interacts with other boys, revealing a conflicted attachment. Events like visits to a public swimming pool with friends, a Christmas gift exchange where Ichika receives a present from him, and New Year celebrations depict his involvement in social activities alongside Ichika and her friends. These situations highlight his gradual, albeit reluctant, integration into the new family dynamic and social group. A few sources list his age as 13, creating a minor inconsistency.