Koji Seki, grandfather and guardian to Kenji Seki, lives with Kenji and his younger sister. He possesses substantial knowledge and capabilities concerning psychic phenomena. In the past, he deliberately sealed Kenji's innate, potent psychic abilities to protect him. Koji Seki is aware of time traveler Ryoichi Kyogoku's existence and activities. Upon encountering Ryoichi, he recognizes him and reveals a prior connection, having known Ryoichi's parents in the past, indicating a history of conflict or disagreement with them.

When Ryoichi's actions escalate at the school and Kenji confronts the crisis, Koji Seki intervenes directly. He unseals Kenji's psychic potential, enabling Kenji to fully utilize his abilities. Later, during events involving Kenji and Ryoichi's journey to the future and their attempts to return, Koji Seki expends considerable energy. He uses his powers to aid their return to the correct time period, an exertion that results in his collapse.