Okō lives in Edo's Fukagawa district during its historical period. After her original family perished in a fire, she was adopted into the family of her younger brother, Seiji. Together they own and operate the Izumo-ya, a lending shop specializing in household items, where she manages operations and client interactions.

She possesses practical intelligence, notably recalling past solutions to business challenges. For instance, she remembered transforming an inexpensive comb into significant profit to help the Omi-ya young master recover a seal case held for ransom. Her methods often involve leveraging the abilities of the shop's tsukumogami—sentient objects—deploying them to discreetly gather information during rentals, uncovering secrets or locating missing items like the netsuke in the Rikyuu Mouse case.

A significant unresolved matter involves Satarō, heir to the Iida-ya shop. Past interactions include a test by Satarō’s mother, challenging Okō to generate substantial profit from a comb. The nature of their relationship and Satarō’s subsequent disappearance form an ongoing element of her background.

Her dynamic with Seiji is collaborative as they jointly operate the shop and consult on cases. However, episodic mysteries often shift focus toward Seiji’s activities, leading him to take primary action in resolving challenges like the comb profit test. This occasionally results in Okō adopting a more observational role during investigations, such as witnessing the extortion of the Omi-ya young master without immediate intervention.