Nui shares a spot of the Four Heavenly Kings with his twin sister Veil. He is an idol like her and they use their concerts to gather people and steal energy from them.

Nui cares deeply about Veil and has sworn to always be with her. He thus attends St. Cherine Academy's middle school along with her. He believes humans are horrible creatures who always fake smiles and emotions to get by.

He seems to have gotten interested in Sumire after she showed no particular interest in overhearing his negative comments on humans.