Haruhiko Amami, a second-year student at Shimano Gakuen, lives in Narasakicho, a seaside town marked by an eternal rainbow and the "Gift" phenomenon. This miracle allows each resident one magical act in their lifetime, dependent on mutual feelings between giver and receiver. His mother, Haru Amami, passed away soon after his birth. Unknown to most, she created the eternal rainbow as her Gift on his birthday, establishing it as the source of all subsequent town Gifts. This connection fuels Haruhiko's staunch defense of the Gift phenomenon, convinced it brings happiness and crediting it with saving his own life.

He shares a home with his frequently absent father, Souichi Amami, granting Haruhiko significant independence. His childhood friend and neighbor, Kirino Konosaka, helps by waking him (as alarms fail against his heavy sleep) and preparing breakfast despite limited cooking skills. Haruhiko often teases Kirino's timid and insecure nature, behavior she tolerates due to her romantic feelings. He maintains a close friendship with Masaki Edo, who occasionally counters Haruhiko's optimistic Gift views with rational arguments.

Haruhiko's life shifts dramatically with the sudden return of Riko Fukamine, his non-biological younger sister. She joined his family in third grade under unspecified circumstances and left years prior, only to reappear unexpectedly. Now sharing his class, Riko openly holds romantic feelings for him, which he reciprocates but hesitates to admit. Her return rekindles past emotions, complicating his relationships and sparking rivalries, especially with Kirino. Haruhiko displays a selfless and protective instinct, particularly when Riko and Kirino face threats from corrupted Gifts. These distortions manifest as black streaks in the rainbow and cause localized havoc when the required emotional reciprocity is absent.

To resolve the corrupted Gifts endangering Riko and Kirino, Haruhiko makes a critical decision tied to the Gift's fundamental rule: each person can only send or receive Gift once. As the eternal rainbow was his mother's Gift to him, removing it becomes necessary to nullify the distortions. This act erases the source of all Narasakicho Gifts but costs Haruhiko his life temporarily. Riko later discovers and revives him, solidifying their profound bond. The sacrifice underscores his willingness to prioritize others' safety over the phenomenon he cherished.

In other media like the OVA special "Ki no Saka Ryokan Kiki Ippatsu!!," Haruhiko actively supports Kirino in preventing her family's inn from bankruptcy. Their effort involves creating high-class cuisine to rival Hotel Moonstone. During this, Yukari Kamishiro, Chisa Fujimiya, and Rinka Hokazono inexplicably develop intense romantic feelings for Haruhiko, sparking chaotic competition for his attention. Despite the turmoil, he remains committed to aiding Kirino, showcasing his loyalty amidst recurring interpersonal complications.

Haruhiko presents an outgoing and cheerful personality, engaging in playful antics and jokes, often at Kirino's expense. Beneath this lighthearted exterior, he is fundamentally kind, helpful, and deeply values his friends. His protective instincts surface when those close to him face danger, prioritizing their safety above his own interests, exemplified by his Gift sacrifice.