Nyu is a split personality of Lucy that developed after a .50 BMG round pierces the metallic helmet encasing her head. Nyu has a childlike personality and infantile knowledge of the world, and lacks spoken language skills, being able to say only "Nyu" and "Kohta" most of the time. Near the end of the anime, she develops a wider vocabulary, with the ability to say phrases like "Don't cry, Kohta!". Nyu is innocent and incapable of violent acts, a foil to the normally cold and sadistic Lucy; she is the manifestation of her "good side". When Nyu is attacked violently (or hit on the head), she regresses into Lucy; likewise, when Lucy is treated with love and kindness, or wants to hide herself from Kohta, she will change back into Nyu. While Nyu exists first due to trauma, it is believed that Lucy subconsciously encourages her presence due to her feelings of guilt towards Kohta and to prevent her from harming him (directly or indirectly). In the manga, this is confirmed by Lucy herself.
During the final episode, Nyu cuts her hair to try to remind Kohta of them when they were younger and apologizes for killing his family— coincidentally, this is the same hairstyle Lucy donned in her youth. She is subverted by Lucy for a larger portion of the manga's final volumes, but reemerges in chapter 106, helping her other personality stop a third personality, based entirely on Diclonius instincts, from harming Kohta. She then urges him to kill her and fulfill the promise he made with Lucy so long ago, which he does. After her death, Kohta names his daughter after her.
