Rikka Amatsuka is the seventh daughter in the large Amatsuka family, one of nineteen sisters. This 12-year-old attends junior high school as a first-year student and stands 148 cm tall. Outgoing and cheerful, Rikka actively seeks closeness with her newly adopted older brother, Yotaro, expressing clear romantic interest in him. Her personal interests center strongly on fashion, makeup, and dressing up, driven by her desire to appear mature. Within the family, her interactions include sisters with differing attitudes towards Yotaro. Rikka's comparatively developed physique, particularly her bust size, becomes a point of contention with Tsurara, the sixth daughter, who expresses jealousy despite being older. This physical attribute occasionally surfaces in sibling comparisons. During a family outing, Rikka participates in a promotional video project to bond with Yotaro. In one incident, she and Hotaru, the fifth daughter, attempt to demonstrate maturity by performing an idol dance in mini-skirts for him. Rikka actively pins Yotaro to the floor during this performance, declaring she is "not a little girl anymore," before Tsurara interrupts. This behavior highlights her persistent effort to gain Yotaro's romantic attention and validate her self-perceived maturity. Her background remains defined by her role within the large familial structure. Relationships primarily revolve around her affections toward Yotaro and casual rivalries or comparisons with sisters like Tsurara.