Katsuhiko Muraki is related to Kasumi Kishimoto by marriage as the nephew of her aunt Chigusa's late husband. He shares a long-term romantic relationship with Kasumi prior to the narrative's events. Early in the story, he leaves Japan for the United States, accompanied to Narita Airport by Kasumi and Yusaku Takasugi for his departure. After two years abroad, he returns to Japan as a professional racing driver. His return serves dual purposes: competing in a racing event and intending to marry Kasumi. During an earlier summer vacation in Japan before his professional career, he initially planned to propose to Kasumi. Upon observing Kasumi's developing feelings for Yusaku Takasugi, a tenant at the Hidamari Private Boarding House, he chooses not to propose. Instead, he challenges Yusaku to a "love game," explicitly framing their rivalry around winning Kasumi's affection. This action confirms his awareness of the changing emotional dynamics and his decision to address the situation directly rather than pursuing his original marriage plan.