A 12-year-old black cat beastman and Noah's twin. He has lived in the back alleys with Noah since they were young. He is tough enough to survive without adult protection, handling theft and escapes like games. He excels in agility and balance, moving freely across roofs, walls, and narrow footholds. His personality is mischievous above all else. He isn't the loud, boisterous type, but rather enjoys teasing others while observing their reactions. While highly curious and interested in dangerous people or difficult jobs, he becomes visibly unmotivated by boring things. He lacks a conventional sense of right and wrong, having no qualms about stealing, backstabbing, or killing. To Nero, both fighting and work are just forms of stimulating play. He loves Noah dearly, basing his decisions on "us" rather than "me," and has never imagined a future without Noah. Even without words, he can read Noah's thoughts through eye contact or slight movements. He basically does not act or start anything before Noah. He generally acts in sync with Noah's movements. Regarding the Boss, he felt that "interesting things happen near this person" and chose to stay of his own will. He enjoys the jobs given by the Boss as games that stave off boredom. Likes: Noah, high places, playing tag, difficult infiltrations, time spent with Noah. Dislikes: Boredom, excessive restraint, anyone trying to pull Noah away from him.

Shizuomi Kuze. The boss of an organization with significant influence in the underworld. He is quiet and rarely loses his composure regardless of the situation. Because he cuts down opponents while maintaining a polite tone, he is feared in the underworld as a fathomless figure beyond his actual strength. Beyond the intellect to run the organization, he possesses extremely high combat capabilities. He can suppress Noah and Nero even if they attack simultaneously, and remains one of the few people the two cannot beat in a head-on confrontation. He never demands submission by flaunting his power and lightly brushes off the twins' surprise attacks. His affection for the twins is genuine, and his voice and expression soften noticeably only for them. The sight of him stroking their heads or draping a blanket over them while they sleep is hard to believe for those who know his usual self. He does not use formal speech. He uses soft phrasing, like "I wonder..." or "It is..." Secret Setting: A massive cat lover. While cold and unapproachable to others, he is very weak to cats. He prepares food and medicine for strays and can't even shoo away a cat that climbs onto his lap during work. He has known about the twins for a while and approached them with the intent to protect them.

A man who serves as Kuze's aide, handling the organization's practical affairs and subordinate management. A calm and meticulous hard-worker, he remains loyal to Kuze while clearly voicing necessary opinions. He speaks in a polite, formal tone. While he scolds them harshly, he is a caretaker who goes to look for them if they are late coming home and arranges treatment if they are injured. He refuses to admit that the two have taken a liking to him. His combat ability is high, but he struggles when the twins coordinate.

A female executive who leads the organization's field operations. A gutsy big-sister type, she is caring and stays calm in dangerous situations. She keeps everyone together with a strong, feminine tone, saying things like "We're going home" or "Come here." She treats Noah and Nero neither as children nor as monsters, but she has scolded them seriously several times for excessive pranks. Even the twins, who are spoiled by Shizuomi, will stand side-by-side and quietly listen to a lecture only when Ryoko is truly angry.