Unkai Saitō is the biological father of Yakumo Saitō, conceived when he raped Azusa Saitou. Both of Unkai’s eyes are crimson red, a trait inherited by Yakumo only in his left eye. Unkai possesses the ability to see ghosts and spirits, similar to his son. He died approximately two to three years before the series' events and exists as a spirit of the dead. As a ghost, Unkai cannot directly cause physical harm, so he manipulates the emotions of living individuals to instigate crimes and fulfill his objectives.

Characterized as cruel, devilish, and indifferent to others' feelings or opinions, Unkai views all human nature as darkness. He employs any means necessary to achieve his goals, treating people as expendable tools. His primary objective is possessing Yakumo’s body, believing Yakumo to be the only compatible vessel due to their shared red eyes. This possession requires Yakumo to be consumed by hatred and despair. To facilitate this, Unkai orchestrates events to emotionally break Yakumo, including manipulating others to target Yakumo’s loved ones. He influenced Sakakibara to murder Isshin Saitou, Yakumo’s uncle and primary father figure, and plotted against Gotou, Yakumo’s guardian.

Unkai’s background reveals complex familial ties. He is the son of Rin Kawakami and stepson of Harunobu Kawakami. His half-siblings include Azusa Saitou, whom he raped, and Isshin Saitou. Beyond Yakumo, his biological children include Yuuta Takagishi. Nao Saitou is identified as both his daughter and niece. Despite these connections, Unkai harbors intense hatred and operates without remorse. His plans ultimately fail when Yakumo proves emotionally resilient, preventing their souls from aligning for possession.