Kyousuke Yotsuya, Miko Yotsuya's younger brother, has short black hair and brown eyes. He is shorter than his sister and favors casual clothing often featuring animal designs. Kyousuke lives with his mother and Miko following their father's death prior to the story.

He exhibits a strong protective instinct toward Miko, assuming she might have a boyfriend and taking it upon himself to watch over her, sometimes jumping to conclusions about her personal life. Their relationship reflects typical sibling dynamics, including shared childhood experiences like bathing together. Kyousuke can display shyness, such as when questioned by a shop employee while buying their mother a birthday present.

Initially unaware of Miko's ability to see terrifying spirits, Kyousuke later develops the same power to perceive supernatural entities. This leads him to join "Sora's team," collaborating with Miko to confront spiritual threats while hiding their abilities from their mother. His protectiveness extends to this new role; he becomes particularly defensive when the spirit Kikuri annoys or harasses Miko, frequently clashing with Kikuri to shield his sister. Other supernatural allies recognize these motivations despite his occasional mischievous actions.

A significant incident highlighting Kyousuke's vulnerability occurred when he interacted with a possessed snowman decoration, unaware of the danger. The attached spirit nearly froze him to death. Miko intervened by destroying the snowman with a can of hot red bean paste and carried him to safety while pursued by the vengeful spirit.

The characters in Kyousuke's name – "恭" (kyou) meaning "respectful" or "polite," and "介" (suke) meaning "help" or "assist" – align with his polite demeanor and helpful actions toward Miko and stray cats.