Nagi is a tsukumogami, a spirit born from a katana that has manifested a humanoid form. He dwells within the Nagatsuki household among other benevolent spirits, serving as a guardian under Botan Nagatsuki's stewardship. His entire existence revolves around protecting Botan, whose safety is further ensured by a neutrality law shielding her from attacks by Saenome factions. While Nagi outwardly maintains a human appearance, he possesses the ability to transform his hands into functional katana blades. This ability directly reflects his nature as a weapon spirit. In combat, he consistently demonstrates restraint by deliberately using the blunt sides of his blades to minimize injury. Upon first encountering Hyoma Kunato, who openly expressed hatred for tsukumogami, Nagi displayed immediate hostility. He instigated a physical confrontation by transforming his hands to attack Hyoma. However, after the collective efforts of the household spirits subdued Hyoma, Nagi expressed regret, acknowledging the use of excessive force. Safeguarding Botan alongside the other household spirits remains Nagi's primary role and duty, forming the core of his actions and relationships within the Nagatsuki residence. Information regarding his origins prior to becoming a tsukumogami, specific personal history, or significant character evolution beyond his established protective role and the resolution of his initial conflict with Hyoma is not provided.