Sai Nanohana is the father of protagonist Jiyu Nanohana. He works as a ghostwriter for samurai novels from home, a career chosen after his wife died while he was at his office, leaving Jiyu alone during her mother's final moments. This tragedy fueled his guilt and his commitment to be a present father.

He shares a close, affectionate bond with Jiyu, often calling her "Jubei." During the second series, Jiyu encourages him to write original romance novels under his own name. He tries but secretly reverts to ghostwriting samurai dramas for his former employer, causing familial conflict when he hides this decision.

Initially providing comic relief in the first series—often mistaken for a stalker and suffering mishaps—Sai possesses hidden competence. He taught the swordsman Yagyu Jubei the defensive "Yagyu Shinkageryu Mutodori" technique for disarming opponents without injury. Sai later demonstrates this skill himself, disarming Mick and restraining him.

Learning of Jiyu's transformation into Yagyu Jubei and her battles through Mikage Tsumura, Sai confronts antagonist Taiko Daiyu. He allows himself to be possessed to fight Jiyu directly, intending to protect her despite the risk, and later aids her in defeating Taiko.

In the second series, Sai develops renewed romantic feelings for Mikage, now his supervisor. Jiyu discovers his lingering affection and reacts negatively. Freesia Yagyu exploits Sai's vulnerabilities, manipulating him into believing he failed as a father. This leads Sai to slap Jiyu in a moment of distress upon her return home, triggering her temporary memory loss and emotional confusion.

Remorseful, Sai actively participates in Jiyu's recovery, helping restore her memories and resolve. He supports her decision to confront Freesia and the Siberian Yagyu threat. After these events, Sai continues his writing career under Mikage's professional supervision.