Kazuma Hiyama appears as a delinquent-like prankster in Class 6-2, characterized by impulsive remarks and foolish antics. He joins a group of boys teasing girls, primarily focusing on Yui Aoi. This behavior conceals his true feelings; he developed a crush on Yui in fifth grade after seeing her cry over her absent mother on Mother's Day. Witnessing her later smile triggered a confrontation where he told her not to smile when sad, creating mutual animosity. Despite Yui's resulting dislike, his affection endured, expressed through persistent teasing.

A pivotal shift occurs when Kazuma helps Yui recover her mother's precious lost necklace. This act reveals his underlying kindness and dedication, leading to a mutual confession. They begin dating, becoming the second couple in their class. Kazuma's personality transforms significantly: he sheds his troublemaker persona, becoming protective and caring towards Yui. He stops his disruptive behavior and grows more considerate.

Physically, Kazuma has chalky brown hair and dark purple eyes, often shown in school uniform. His background notes an undisclosed family situation, specifically mentioning his mother as unknown. His name combines "one" (一) and "soar/fly" (翔) for "Kazuma," while "Hiyama" means "hinoki cypress" (桧) and "mountain" (山).

His relationship with Yui remains central throughout the series, defined by protectiveness and emotional support. Early conflicts, born from misguided attempts to gain her attention, evolve into a sincere bond of mutual care after the necklace incident. His interactions broaden to include classmates like fellow pranksters Eikou and Tomoya, with whom he previously engaged in teasing and popularity rankings before his personal growth.