Katsuhiko Muraki is Kasumi Kishimoto's boyfriend when the story begins, living abroad because of his parents' work. He is the son of Aunt Chigusa's late brother-in-law, sharing no blood relation with Kasumi despite this family tie. He possesses good looks, excels academically, and demonstrates strong athletic ability. He maintains a long-distance relationship with Kasumi while attending UCLA in California.

During a summer vacation, he returns to Japan intending to propose to Kasumi. However, he witnesses her growing emotional attachment to Yusaku Takasugi, another resident of the Hidamari Private Boarding House. Acknowledging this shift in her feelings, he chooses not to propose. Instead, he issues a dignified challenge to Yusaku, initiating a competition for Kasumi's affection. In the anime adaptation, he becomes more involved with the boarding house residents by taking on the role of manager for the Myojo High School baseball team during the Koshien district qualifying finals.

Two years after his initial departure, he returns to Japan as a professional racing driver. This later return is driven by his continued desire to marry Kasumi, leading to further developments in their relationship. His presence consistently acts as a catalyst, prompting exploration of Kasumi's conflicted loyalties between her childhood connection to him and her evolving feelings for Yusaku.