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.
Nui and Veil are originally Matryoshka dolls with souls, who lived with an old woman in Russia. After her death, the villagers tossed them away, considering them cursed. Mary later picks them up and turns them into androids.
His wish is to become human, due to his desire to be by his sister's side in the same form as her, and so he won't be hated by people again. For that reason, he steals energy from humans and turns it into coins so that Mary can eventually use them to grant his wish.

At first, he tries to persuade Veil to stop getting friendly with Meguru because he believes Meguru would start hating Veil if her secret of being an android was revealed. After Meguru decides to keep being friends with her, despite learning her secret, Nui keeps trying to separate them, but this time because he discovered Meguru is Angel Rose, their enemy.
He eventually accepts Veil's desire to stay friends with Meguru and is convinced by the later that she was serious about their friendship, even if they are on opposing sides.
At their last concert, he follows Veil's decision to turn against Mary.

He dies while fighting Mary, who activates a switch that makes his body explode.