Hina Tōmi is a classmate of Hime Gotō and serves as the lunch committee member within their school's Meguro River Detective Agency. Her surname Tōmi combines the kanji for "east" (東) and "control, honorable" (御), while her given name Hina is written in hiragana (ひな), though it can carry meanings related to sunlight or vegetables.

She lives in a high-rise apartment building with concierge service, indicative of her affluent background. Hina tends to show off and project a mature image, sometimes making insensitive remarks, such as asking Hime—who lives with her single father—if her family is poor, and openly stating she doesn't believe in Santa Claus.

A visit to Hime's home reveals the Gotō family's display of ten-tiered Hina dolls, a traditional and expensive decoration for Hinamatsuri. This challenges Hina's assumption about their financial status and undermines her self-identity as the "rich girl." Consequently, she avoids discussing Kakushi Gotō's profession and begins to suspect he might be involved in illicit activities to afford such possessions.

This experience subtly alters her interactions, making her more guarded around Hime regarding topics of family wealth and parental occupations.