In the alternate universe of Mai-Otome, Nagi Dài Artai is the ruler of the Principality of Artai. His right-hand man is Major Sergay Wáng of the Artai military. He enjoys teasing and flirting with Mashiro.
Unlike his Mai-HiME incarnation, this Nagi is outright malevolent and more of a warmonger and mastermind. In the beginning of the series, he tries to take over Windbloom Kingdom, first by proposing marriage to Queen Mashiro Blan de Windbloom (who rejects him) and then by force, with the help of the Schwarz organization and its public figure, John Smith.
When Smith retrieves data regarding the lost technology of the Earth colonists, he uses it to mass-produce a new generation of SLAVE units. After Schwarz takes over Windbloom with the help of the SLAVEs and a powerful cannon secretly installed on top of the rebuilt Windbloom Castle, Nagi organizes a staged "liberation", moving the Artai army into the city and putting it under his authoritarian control. Despite the outrageous nature of his actions, other nations do not intervene, as Artai is technically not violating any international laws. Meanwhile, Nagi is shown to have been in possession of the Ultimate Black Diamond GEM all along, which he uses to forge a contract with Nina Wáng, his new Meister Otome.
As the series progresses, it is revealed that Nagi is interested in the hidden technology stored beneath Garderobe Academy and Windbloom Castle. More specifically, Schwarz's research provides him with his own Otome unit, the Valkyries, and unlocking the long-lost Harmonium provides him with a weapon of mass destruction to dictate his will to foreign nations. After an impressive demonstration of the Harmonium's power, some nations do side with Artai, while others — most prominently Aries and Zipang — forge an alliance against Nagi, organized by Natsuki Kruger, the refugee Headmaster of Garderobe. A war erupts.
During the final battle, Natsuki and her Otome retake Garderobe and defeat Nagi's SLAVEs, while Arika Yumemiya puts an end to the Valkyries and later defeats Nina and renders the Harmonium useless; Mikoto then destroys the Harmonium. Despite rapidly losing on all fronts, Nagi tries to retain self-control; he explains that he wanted to throw the society back to the way it was before the advent of the Otome system, which he considers an ineficient substitute for war.
Nagi tries to run away, but is arrested by Chie Hallard and put into servitude to Mikoto the cat goddess along with the remaining Schwarz cultists. At the end of episode 26 he is seen trying to escape, but Mikoto bashes him with her staff, rendering him unconscious, and then drags him back to the chain of prisoners.
He also appears briefly in episodes 1 and 2 of Mai-Otome Zwei imprisoned, the first with a non speaking role, and in the second with a small part at the end of the episode where he suggests that information on the mysterious entity petrifying the Otome may be located in Artai. He is also seen in the last episode of Mai-Otome Zwei looking at a Book called "Nagi's Smile Diary" XI or XII, suggesting that he's been keeping diaries since he's been in prison.