Takumi Sekiya is a third-year high school student. He sports bleached yellow-blond hair styled in a quiff, with the shaved sides revealing dark gray hair. His thick, slanted, and hard-angled eyebrows match this gray color. His typical outfit consists of an unzipped, mixed light blue and white raised-collar jacket worn over a white shirt.

Sekiya persists relentlessly in pursuing romantic interests, continuing his attention even after rejection. He follows Nanako Watase to a mall after she requests time to consider his confession. There, he encounters Kazuhiro Nakamura, whom Nanako identifies as her boyfriend. Sekiya reacts visibly discomforted by both Nakamura's eccentric behavior and his claim that Nanako is his soulmate from a previous life.

His notable involvement occurs in Episode 2, "The Reincarnation of an Angel or a Devil." While others discuss recent unexplained events and potential culprits, Sekiya watches them intently through a glass panel, his breathing noticeably audible. He denies any involvement in the incidents under discussion.

Sekiya is identified as a musician in one source, though no specific details about his musical activities or role are provided. His name combines the characters for "expand, open, support" (拓) and "sea, ocean" (海) in "Takumi," and "barrier, gate" (関) and "valley" (谷) in "Sekiya."