Age: 14 (in appearance)
DOB: April 12
Height: 155 cm
Weight: 47 kg (103.4 lbs)
Blood type: B
Nagi makes his first appearance in the first episode of the My-HiME anime commenting that another HiME has arrived. He is first seen conversing with Mashiro Kazahana and Fumi Himeno after the latter two meet Mai Tokiha for the first time and he comments on Mai's HiME status. Later in the same episode, Nagi appears to Mai and Natsuki Kuga, and implies that her brother Takumi Tokiha is threatened by an Orphan, and as Mai strives to protect him, Nagi introduces her to her HiME powers and her Child, Kagutsuchi.
Nagi is an observer to the awakening of several more HiMEs, and also acts as a sort of referee to prevent fights between them. It is revealed that he is behind the Orphan attacks on Fuka Academy; both Fumi and Mashiro (who mentions Nagi being part of the First District) are aware of this, as is Natsuki, who reluctantly leaves him alone as long as he discloses the Orphans' directions.
Soon, however, Orphans of another kind come into play, catching even Nagi by surprise as he is not aware who sends them. The other Orphan master turns out to be Alyssa Searrs, an artificial HiME created by the Searrs Foundation with the goal of disposing of all other HiME. Nagi, still staying in the shadows, aids the other HiME in their quest to protect Fuka Academy from the Searrs Foundation.
After Searrs's plans are foiled and Alyssa is killed, Nagi appears before the "HiME Rangers", newly formed by Midori Sugiura to more efficiently protect Fuka against Orphans, and tells them that Orphans will not be appearing any more — instead, the HiME will have to fight each other's Childs until only one HiME retains her powers, as part of the HiME Carnival that occurs every 300 years. He is later revealed — along with the First District — to be a servant to the Obsidian Lord, the ancient power behind the HiME Carnival, and that Nagi himself witnessed previous HiME Carnivals, which would make him several centuries old.
As the HiME Star descends to Fuka Academy, Nagi guides Mai (the only HiME remaining by that point, bar Mikoto Minagi) through Valhalla's Gate to the HiME star and to her final showdown with Mikoto and the Obsidian Lord. He then tries to prevent Miyu Greer from following Mai's footsteps into the hall with the pillars representing HiME, but is to stop Miyu unsuccessful, and to his shock, she unleashes her ultimate "Platinum" mode, shattering the crystal in which Mashiro's real body is encased and freeing her.
After the Obsidian Lord is defeated, Nagi's eyes glow yellow and he screams, clutching his head. When he regains consciousness, he laments his failure to prevent the HiME from cheating their fate. He is last seen conversing with Mashiro and Fumi; he comments that the destruction of the HiME Star made the future less certain. Nagi and Mashiro then decide to "go back where they belong" and disappear in a pitch black room behind a massive gate, which then closes behind them, leaving Fumi behind.
Nagi, as portrayed in My-HiME, is a trickster. He likes eavesdropping, standing or sitting on high ground (especially tree branches), and suddenly and unexpectedly appearing before other characters to deliver vague bits of information. He wears a white shirt and usually has his hands put in the pockets on his trousers. He also often carries a book or balances it on his head; in the final episode, just before disappearing, he gives the book to Fumi, and it is revealed to bear the words "Fuka Academy" on its cover.