Yū Kobayakawa attends the Sunflower Class as a second-year kindergarten student. He is the eldest son in a four-member family, where his father holds a diplomatic position. Records list his blood type as O, and hamburgers rank as his favorite food.

Physically, Yū possesses light brown hair that falls to his neck and striking blue eyes. His nature is inherently kind and cheerful. This disposition manifests in actions like assisting classmates; he once helped Koume after she fell, an act that unexpectedly sparked her affection for him. While not attention-seeking, his friendly presence naturally eases Koume's shyness.

He maintains friendships with peers, including Daigo and Haruto. Demonstrating his considerate side, he presents Koume with a stuffed koala doll following the summer break. Details about his family structure and personal preferences were uncovered through Hiiragi's informal background research.