Seiran Shinano acts as butler and guardian to Mayuki Hyuuga in Kyuto, the city rebuilt after Tokyo's destruction in a massive earthquake. Hailing from the Shinano family, long-time supporters of the Hyuuga clan, his responsibilities surpass household management to include covert surveillance of Kyuto's supernatural phenomena from a hidden base. His unwavering commitment originates from a promise to Mayuki's mother, Shara Hyuuga, to protect her son at all costs.

Biologically, Seiran is Mayuki's maternal uncle and brother to Shien Shinano and Seiju Shinano (later Masaki Hyuuga). He initially conceals this familial connection, presenting himself solely as a loyal retainer. His relationship with Shien evolves from early animosity towards mutual understanding, centered on their shared duty to safeguard Mayuki. The Shinano family history holds complex dynamics, notably Seiju's resentment after being displaced as the Hyuuga heir by Mayuki's birth.

Seiran manipulates specialized warriors called Ayas using doll mastery. His primary Ayas are Hatsumi Mieno, transforming into Sōka (who wields her hair as a whip), and Sanae Izumi, transforming into Yūhi (who employs claw weapons). Functioning as both domestic staff and combatants, they activate via specific command phrases. Their floral-themed alter egos—Sōka for Asoka flower, Yūhi for hydrangea—follow a naming pattern seen in other Ayas like Byakko (Polygonatum Odoratum) and Suiren (water lily). To bolster his defenses against threats, Seiran undergoes a perilous Shinano clan procedure involving specialized needles, granting heightened powers at significant personal risk. This upgrade proves vital during clashes with Seiju, who later masters the technique more completely.

His character demonstrates strategic analysis and reserved detachment, often favoring logical solutions over emotional engagement. This approach guides his investigations into Kyuto's supernatural incidents, coordinating Mayuki's protection while obscuring their full familial legacy. Key conflicts emerge when Seiju orchestrates events to awaken Mayuki's inherited Shinchi power, forcing Seiran into direct confrontations. These battles expose his limitations, especially when overpowered by Seiju's perfected techniques, resulting in physical defeats that highlight the threat level. His steadfast dedication to Mayuki's safety remains central throughout, even as his secret familial ties gradually surface.

Visually, Seiran appears as a black-haired adult with brown eyes, typically attired formally in a vest and tie. His operational base contains technology for monitoring Kyuto, reflecting preparedness for emerging supernatural threats tied to the city's phantom cases.