Kazuma Hoshino moves to the countryside town of Yamabiko with his younger brother Ayumu, seeking cleaner air to aid Ayumu's delicate health and asthma. Their parents remain in the city temporarily, planning to join them later, while the brothers stay at Yorozuyo Inn under the care of family friend Senka Yorozu. Fragmented, unclear childhood memories of Yamabiko resurface for Kazuma as flashbacks during his reacclimatization to the town.

Visually, Kazuma appears as an ordinary teenage boy of average height with brown hair featuring spiky bangs. His eye color varies: blue in most depictions, brown in the original visual novel, and violet in some anime references. He typically wears his Yamabiko South Academy uniform—a white shirt, brown tie, and black trousers—or casual attire like t-shirts and shorts outside school.

Kazuma exhibits a fundamentally warmhearted, kind, and polite personality, often displaying gentlemanly behavior. A strong protective instinct toward Ayumu leads him to prioritize his brother's well-being above most concerns, initially manifesting as close oversight that eases as Ayumu makes independent friends. He struggles with decisiveness, particularly in romantic situations, due to a deep-seated desire to avoid hurting others' feelings. His athletic background includes prior tennis club membership, contributing to his physical competence.

Relationships form a core aspect of his experience. He shares a deep familial bond with Ayumu, whose health motivated their relocation. An accidental fall resulting in a kiss with Ui Nakatsugawa at their first meeting triggers rumors labeling him a "playboy" and earns initial distrust from peers like Ibuki Hinata. Over time, his consistent kindness and integrity gradually dispel these misconceptions, fostering mutual respect and friendships. His camaraderie with Daigo Minamikokubaru is marked by the nickname "bro." Romantic developments see him forming connections with multiple female characters, though Ui Nakatsugawa becomes his canonical girlfriend.

Kazuma's development revolves around community integration in Yamabiko. Initial social friction from misunderstandings gives way to acceptance as his earnest efforts and inherent decency win over peers. He navigates evolving romantic feelings while managing his protective role toward Ayumu. The OVA depicts an uncharacteristic surge of intense protectiveness when Ayumu is endangered. Moments of vulnerability occur, such as embarrassment after accidental nudity incidents where his physique becomes a subject of teasing.

His name carries specific meanings: "Kazuma" combines "one" (一) and "horse" (馬), while "Hoshino" means "star" (星) and "field" (野).