Tomomi Mizuno possesses brown eyes and long brown hair, typically secured in a bun without bangs. Her slim physique measures 158 cm tall with an 82-55-82 cm figure. A pale complexion and tareme-shaped eyes enhance her reserved demeanor. She frequently wears glasses and a school uniform, often accessorized with a headband, bow tie, or vest. Other attire includes a haori, miniskirt, turtleneck sweater, yukata, one-piece swimsuit, and towel.

Exhibiting a reserved and intelligent personality, she fits the "watashi" pronoun archetype. As an honor student, she maintains diligent academic performance. Her background establishes her as Ryunosuke's childhood friend and neighbor, sharing a long-standing connection. They attend the same school as twelfth-grade classmates. She comes from a wealthy family.

Within the narrative, she shares a close friendship with Izumi Shinohara. A significant relational dynamic prevents simultaneous romantic involvement between Ryunosuke and both Tomomi and Izumi, indicating narrative exclusivity in potential pairings. Her character is subject to teasing by others. Outside academics, she engages in skiing as a recreational activity.