Masato Wakamatsu is the main male protagonist of the series. He is a high school student who lives with his younger stepsister, Miyuki Wakamatsu, with whom he shares no blood relation. Their father works overseas, leaving them to live alone in Tokyo. Masato’s background is marked by the sudden return of his sister after six years abroad, which upends his daily life.

In terms of personality, Masato is often characterized as lazy and academically struggling, in contrast to his sister’s studious nature. He can be impulsive and prone to misunderstandings, especially in romantic situations. Despite these flaws, he shows care and protectiveness toward his sister and his classmates. His motivations revolve around navigating his feelings for two girls who share the same name: his classmate Miyuki Kashima, whom he dates, and his stepsister Miyuki Wakamatsu, toward whom he develops conflicted romantic and familial emotions.

His role in the story is central, as the series follows his romantic and domestic entanglements. Key relationships include his girlfriend Miyuki Kashima, who is pretty and gentle, and his stepsister Miyuki Wakamatsu, who is cute, sensitive, and popular. He also deals with friends and rivals who pursue his sister, such as Ryuichi Masaki, and his girlfriend’s father, Yasujiro Kashima.

Over the course of the series, Masato matures as he confronts his feelings and learns to balance his responsibilities. A notable ability or trait is that he suffers from a condition known as the “Miyuki Illness,” a psychosomatic ailment caused by going too long without seeing his stepsister. This underscores his deep attachment to her. He also demonstrates resourcefulness, such as when he deduces his sister’s predicament and returns home early to protect her from unwanted visitors. Overall, Masato is a relatable teenager caught in a love triangle that tests his emotions and growth.