Nui is a supporting antagonist in Twin Angels BREAK. He and his twin sister Veil are two of the Four Heavenly Kings who serve the main villain Mary. Nui is a middle school boy who attends St. Cherine Academy alongside his sister, and the pair work as idol performers. Their concerts serve a sinister purpose: they gather large crowds and then drain the audience's life energy, which is converted into coins that Mary can use to grant wishes.

Originally, Nui and Veil were not human. They were Matryoshka dolls imbued with souls, living with an elderly woman in Russia. After the woman died, the villagers believed the dolls were cursed and threw them away. Mary later found them and turned them into android bodies, giving them a human appearance and the ability to interact with the world. Nui's deepest wish is to become a real human. He desires this both so he can stay by his sister's side in the same form as her and so that he will no longer be hated and discarded by people. To that end, he actively participates in Mary's energy‑harvesting scheme, hoping that Mary will eventually grant his wish.

In terms of personality, Nui outwardly appears calm and accommodating, with a strong sense of service. He can come across as pleasant and friendly. However, this surface demeanor hides a cynical and guarded inner nature. He believes that humans are horrible creatures who always wear fake smiles and conceal their true emotions to get by. This negative view of humanity stems from his own painful experience of being abandoned. Despite his cynicism, he shows genuine care and protectiveness toward Veil, having sworn to always be with her. He is also perceptive and becomes interested in the protagonist Sumire when she shows no particular reaction to overhearing his cynical remarks about humans, suggesting he is drawn to authenticity.

Nui's role in the story is that of a secondary antagonist who initially works against the Twin Angels. He and Veil use their idol status to infiltrate the school and collect energy. His key relationship is with Veil; he is fiercely protective and at first tries to persuade her to stop befriending Meguru (one of the Twin Angels) because he fears that Meguru would reject Veil if she learned Veil was an android. After Meguru accepts Veil despite knowing the truth, Nui shifts his efforts to separating them because he discovers Meguru is Angel Rose, their enemy. Over time, he is swayed by Veil's genuine desire to stay friends and by Meguru's sincerity, eventually accepting that their friendship can exist even on opposing sides. This marks a turning point in his development, as he begins to question his allegiance.

At their final concert, Nui follows Veil's decision to turn against Mary. During the ensuing battle, Mary activates a switch that causes Nui's android body to explode, killing him. His death is a direct consequence of his rebellion against his creator. His abilities are not extensively detailed, but as an android he possesses superhuman physical capabilities, and he and Veil can use their idol performances as a means to drain life energy from humans. He also demonstrates cunning and strategic thinking in his efforts to manipulate situations for his and Veil's benefit.