Takayuki Utsunomiya is the son of actress Mitsuki Utsunomiya and becomes Mira Munakata's half-brother after Mira discovers Mitsuki is his biological mother. He attends the same high school as Mira as a third-year student and student council member, recognizable by his blonde hair, yellow eyes, and gakuran uniform.

Takayuki pressures Mira to facilitate a marriage between Kyōsuke Munakata and Mitsuki, aiming to boost his mother's career and status. He employs manipulative tactics, including persistent romantic advances towards Mira. He uses the nickname "Koneko-chan" for her while applying emotional pressure to sway Mira's decisions about Kyōsuke.

Within the extended light novel series, he evolves into Mira's third consummated love interest, developing a more complex romantic dynamic. His efforts to orchestrate Kyōsuke and Mitsuki's union intensify, heightening Mira's emotional turmoil and positioning him as an antagonistic force. The OVA adaptation, covering only the first novel, omits his potential familial reconciliation or deeper relationship developments with Mira.

His actions directly impact Mira's conflicts concerning his relationship with Kyōsuke and the revelation of his adoption.