Yuuto Toudou is a teenage student residing alone with his stepsister, Erika Toudou, following their father's relocation to Sapporo for work. Both of Yuuto's biological parents are deceased; his mother died during his childhood, and his stepmother (Erika's biological mother), who treated him kindly, also passed away, leaving the non-blood-related siblings unsupervised.

Yuuto manages household responsibilities as Erika lacks domestic skills. At school, male peers frequently approach him seeking information about the popular Erika, fueling speculation about their inappropriate relationship. Yuuto develops romantic and sexual feelings for Erika, partly because she resembles his deceased stepmother, whom he misses deeply. He also exhibits a pronounced fixation on large breasts, which can override his judgment when encountering women possessing this trait.

Their relationship evolves into a sexual one while they cohabit. Despite Erika's loyalty, Yuuto becomes involved with his art teacher, Sayaka Ichinose, after she discovers his affair with Erika. This creates a love triangle, with Yuuto engaging sexually with both women. Ultimately, all three agree to a non-exclusive arrangement where both women continue their relationships with Yuuto while maintaining a rivalry over him.

Physically, Yuuto has short brown hair reaching his ears. Sources differ on his eye color, describing them as either gray or brown. He is depicted as a teenager with no other distinctive physical markers.