Uesugi Nagi is the childhood friend of Kai Misonou, having remained by his side ever since Kai was separated from his twin brother, Shou Hamura. Nagi made a promise to protect Kai and stay together with him when they grew up. He serves as Kai's only friend and is potentially in a relationship with him.

Nagi first appears near the end of the initial episode, vowing to protect Kai from anyone who causes him harm. This deep-seated protectiveness fuels a strong hatred within Nagi toward Shou for the pain he inflicted on Kai.

In the second episode, Nagi is seemingly killed while shielding Kai from his own attack, after Kai falls under Ran's control. He survives the encounter but is left with amnesia. During this incident, Nagi awakens as a black-winged entity, an awakening that positions him in direct opposition to the white-winged clan.

Nagi possesses cream-colored, latte-like hair and sky blue eyes. He is frequently depicted chained to a bench.