Seiji is Okō's non-biological younger brother, adopted after a fire displaced her; she was fostered by his birth parents. Together, they run Izumo-ya, a lending shop in Edo's Fukagawa district specializing in everyday items. Some items are tsukumogami—objects inhabited by spirits after a century of existence. Both siblings see and hear these spirits but maintain an unspoken agreement avoiding direct communication. They strategically lend tsukumogami to customers, enabling the objects to gather information that helps solve local mysteries.

Clever and methodical, Seiji relies on tsukumogami gossip and observations to solve clients' problems, like finding lost heirlooms or resolving conflicts. His reputation as a mystery-solver is well-established. One persistent mystery involves Satarō, a man who pursued Okō despite being engaged elsewhere.

Over time, Seiji becomes the shop's primary investigator, taking on more case responsibility. His relationship with Okō is central, marked by latent romantic feelings complicated by their non-biological sibling status. This creates internal tension, making emotional vulnerability—such as giving compliments or discussing personal topics—difficult; he often avoids it. The tsukumogami occasionally mock this hesitancy.

His backstory involves a family upbringing that prized discretion and subtlety, shaping his reserved personality. The adoptive family structure and shared trauma from past Edo disasters like fires underpin his pragmatic outlook. While his investigative skills grow, emotional guardedness persists, particularly regarding his feelings for Okō and adoptive family history.

Seiji's resourcefulness shines in interactions with tsukumogami. He understands their rule of avoiding direct human acknowledgment and exploits it, eavesdropping on their conversations for insights. This indirect collaboration highlights his adaptability and observation skills, though his development centers more on his professional role than supernatural aspects.