Ryoto Mitsuka is a friend of Kazushi from the Serimiya household, presenting a handsome and polite exterior during visits that suggests good manners. This surface behavior conceals a hidden nature described as a "secret personality." His presence disrupts the Serimiya family's domestic routine, particularly affecting Yoko Serimiya, who remains unaware of his undisclosed tendencies at their initial meeting.

Belonging to the Mitsuka family, Ryoto has a relative named Misao Mitsuka, likely his sister. He functions as an agent of change within the family dynamic, with his hidden nature directly challenging the household's peace and stability. This contrast between his outward politeness and disruptive concealed traits becomes a central catalyst for the story's events.