Keiju Tabuki teaches science and advises the birdwatching club at West Garden High School, instructing siblings Shoma and Kanba Takakura. Born in 1988, he is approximately 23 during the main events, with tanned skin, dark plum hair featuring bangs covering his right eye, blue-gray eyes, and oval glasses.

Abandoned as a child after his mother remarried and bore a sibling who surpassed his piano skills, he formed a close elementary-school friendship with Momoka Oginome. Oversleeping on March 20, 1995, he missed the subway attack that killed her, attending her funeral in stunned disbelief—a trauma that haunted him. He maintained bonds with the Oginome family through regular "Curry Days" dinners and knew Momoka's younger sister Ringo from birth.

Outwardly lighthearted and naive, Tabuki concealed torment and vengefulness beneath. He became engaged to Yuri Tokikago, a renowned Sunshiny Theatre Troupe actress drawn to his childlike demeanor. Their engagement announcement at a student-attended party preceded his move into her apartment, but their relationship unraveled over shared, unresolved grief for Momoka. They canceled the marriage upon acknowledging their inability to forge a "fake family."

Ringo Oginome fixated obsessively on Tabuki, convinced she must replace Momoka in his life. Unaware of her stalking and delusions, he treated her solely as a sister. Ringo escalated to attempted rape and sperm theft, which he rejected. After she renounced her obsession and apologized, he forgave her.

Following the broken engagement, Tabuki resigned from teaching. He ultimately reconciled with Yuri when both grasped why they felt "left behind," choosing to remain together.

In film adaptations, his background and arc align with the original series, centering on his trauma, fractured relationships, and reconciliation.