Seiichirō Jingyōji is a young man with short black hair and red eyes who maintains a stoic, reserved demeanor, making his emotions difficult to discern. He voices opinions more freely among childhood friends, particularly Natsusa Yuzuki and Ibuki Ueoka, and occasionally teases Natsusa.

Natsusa holds profound significance for Seiichirō. He began playing rugby because of Natsusa and initially considered quitting after Natsusa's injury ended his own play. Seiichirō persisted solely because Natsusa liked seeing him score. He exhibits strong protectiveness over Natsusa, worrying about his post-accident health, cautioning him against overexertion as team manager, carrying his backpack, and visiting him regularly. Seiichirō supports Natsusa's decisions even when disagreeing. This protectiveness manifests as unexpected possessiveness; he declares breaking certain promises with Natsusa unforgivable, an intensity surprising to others. Natsusa remains the only person who reliably understands Seiichirō's feelings.

As the fullback (number 15) for the Doshisha University rugby team, Seiichirō's performance is intrinsically linked to Natsusa. Watching a game, Natsusa witnessed Seiichirō miss a goal due to nervousness stemming from a fear of disappointing him—a lapse contrasting his usual composure and rare since his second year of high school.

Seiichirō displays rare emotional vulnerability tied to Natsusa, such as physically supporting him and asking if he is happy, briefly smiling at Natsusa's affirmative reply. Others, like teammate Madoka Hongō, sometimes elicit reactions from Seiichirō by pinching him or mentioning Natsusa.