Sefuru is the youngest surviving member of the Oni Clan, around fourteen or fifteen years old. He possesses mixed heritage, with a human mother from Kyou and an Oni father. His mother abandoned him in early childhood because of his physical Oni traits, an event planting deep bitterness towards the people of Kyou and a pervasive fear of abandonment.

Akuram, the Oni Clan leader, took Sefuru under his care and trained him to advance the clan's objectives. Despite his half-human lineage, Sefuru gained acceptance within the clan due to Akuram's influence. His loyalty stemmed from a desperate desire for belonging, though insecurity and fear of isolation often drove his actions.

Sefuru formed an initial bond with Shimon Nagareyama, one of the Eight Guardians, mistaking Shimon's blond hair and blue eyes—traits resembling Oni—for proof of shared heritage. He saw Shimon as a potential ally and confidant. However, discovering Shimon's allegiance to the Dragon Priestess Akane and his role as a Guardian felt like betrayal to Sefuru. This fueled his resentment, leading him to manipulate their friendship in attempts to undermine Akane and the Guardians.

His character shifts from aggression to vulnerability. After suffering grave injuries and being discarded by Akuram for mission failure, Sefuru was taken in by Seri—Inori's sister—and her partner Ikutidaru. Their compassion provided him a stable familial environment, easing his loneliness and reducing his hostility. The narrative concludes with Sefuru integrated into their household, symbolizing his transition from vengeance to peace.

This outcome is confirmed in the manga continuity, while the anime adaptation implies a similar resolution through context.