Riki Kuramoto is Momoko Kuramoto's twin brother. Both twins were born prematurely. While Riki developed normally, Momoko experienced physical and mental disabilities caused by progressively stiffening muscles. This difference means Riki attends a regular elementary school, while Momoko initially attends a special school.

Riki harbors complex emotions about his family life, including jealousy and resentment stemming from the disproportionate attention his parents devote to Momoko. Her frequent hospitalizations force him to stay home alone, fueling frustration over perceived parental neglect.

A significant shift occurs when Momoko joins Riki's regular school on a weekly trial basis. Her presence creates social challenges for Riki, particularly hostility from a classmate, Ryuji Kita, who bullies both siblings. This forces Riki to navigate peer relationships complicated by his sister's disability.

Throughout these experiences, Riki undergoes gradual emotional growth. His initial resentment coexists with a protective instinct toward Momoko, especially evident during confrontations with Ryuji. The family soothes Momoko during distress by singing "The Song of Frogs," a practice Riki participates in, showing his involvement in her emotional support despite internal conflict. Interactions with Momoko at school contribute to his evolving understanding of her struggles and desires for normalcy.

Riki's relationship with his parents remains strained by their focus on Momoko's needs. However, his actions demonstrate a growing, albeit conflicted, commitment to his sister's well-being within both the family structure and the school environment.