Kyōsuke Yotsuya is Miko Yotsuya's younger brother, residing in the Yotsuya household with his older sister and their mother, Tōko Yotsuya. Their father, Mamoru Yotsuya, passed away before the story begins. Kyōsuke possesses short black hair and stands shorter than Miko. He favors casual attire often featuring animal motifs. His personality shows a strong protective streak toward his sister, leading him to speculate about potential boyfriends in her life and vigilantly watch over her, sometimes jumping to conclusions. Despite this protectiveness, their relationship follows typical sibling patterns, including shared childhood activities like bathing together. He also displays moments of shyness, evident when questioned by a shop employee while selecting their mother's birthday gift.

Kyōsuke remains completely unaware of Miko's ability to see spirits or the supernatural dangers she encounters. His focus remains squarely on ordinary family life. His given name, Kyōsuke (恭介), meaning "respectful, polite" (恭) and "help, assist" (介), reflects his generally polite demeanor and helpful nature, especially towards Miko and stray cats. The family surname, Yotsuya (四谷), combines "four" (四) and "valley" (谷). His role centers exclusively on familial interactions within the home, with no documented character development, involvement in supernatural events across the manga or anime, or appearances in spin-offs or expanded media.