Mizuki Shiiba is a supporting character in the anime Junjo Romantica 3. He is twenty-five years old and works for the Usami Group, the family's general trading company. Mizuki is Akihiko Usami's cousin from his father's side of the family. He returns to Japan from the United States and, without invitation, moves into Akihiko's apartment, much to Akihiko's annoyance. Mizuki has a confident and somewhat pushy personality, often inserting himself into situations with little regard for others' convenience. Despite his easygoing exterior, he has a strong attachment to Akihiko and can appear cold and unwelcoming toward Misaki Takahashi, Akihiko's partner. This coldness is not due to hostility toward Misaki personally but stems from Mizuki's own romantic feelings for Kaoruko Usami, Akihiko's sister. Mizuki and Kaoruko frequently argue and bicker, but it becomes clear that his underlying motivation is his affection for her, which he struggles to express directly. In the story, Mizuki serves as a secondary character who adds comic tension through his freeloading presence and his contentious yet heartfelt dynamic with Kaoruko. He does not have any special abilities; his role is grounded in everyday interactions and unrequited romance. His development is subtle, centered on his gradual, reluctant acknowledgment of his feelings for Kaoruko and his adjustment to the relationships within the Usami household.