Nagi is a tsukumogami, a spirit inhabiting an aged object, residing within the Nagatsuki household alongside Botan Nagatsuki and other benevolent spirits. He serves and protects Botan as one of the household's six tsukumogami guardians.

Upon Hyoma Kunato's initial arrival, Nagi reacted with hostility after hearing Hyoma express hatred toward tsukumogami and accuse them of exploiting Botan. During this confrontation, Nagi transformed his hands into katanas, specifically noting he would use the blunt side against Hyoma. When Hyoma nullified parts of the blades, the conflict escalated until other household members intervened and subdued Hyoma. Nagi later remarked he felt some sympathy for Hyoma after witnessing the group attack.

Nagi participated in a mission alongside Kagami in town, recalling Hyoma's scheduled arrival that same day. Later, when the powerful tsukumogami Kai attacked Hyoma, Nagi joined others from the household to intervene and stop the assault. This intervention allowed Kai to explain his actions as a test of Hyoma's capabilities.

No further details exist regarding Nagi's origins, specific object form, broader background story, character development beyond these events, or involvement in spin-offs or other media adaptations.