The main character of the story. He is an unusually naïve boy as proven when he first thinks that blood is just water that has been coloured red and when he believes in anyone who says they can help him. He appears to be 12-14 years old. He claims to have always been living with a man named Karoku in a forest near Karasuna. When Karoku does not appear one day, Nai ventures outside and finds a trail of blood leading from their home to the sea, and a left-behind bracelet . Feeling scared and lonely, he sets out to try finding Karoku with only the abandoned bracelet as a clue. He eventually meets Gareki, who saves Nai from a mansion where a Varuga woman named Mine was keeping him captive. This marks the beginning of their travel together. Despite his unworldly ways, he has an acute sense of hearing that is more well-developed compared to other people's. Akari, a research doctor at Circus reveals that Nai has the cellular structure of a human and a "niji", a small, delicate creature found in the forest he came from, and that these cells have managed to coexist unlike the Varuga. He also said that Nai is successfully existing as a new species and that if they were to describe the person who accomplished this, they would certainly be called a genius and that to produce a being like Nai, the creator must have been very devoted. Nai's name means 'nothing/not existing' when translated to English.