Lucy is a diclonius girl, around eighteen years old and the central protagonist of the series. Lucy has four vectors with a limited range of about two meters. However, she can be swift and lethal within that range, and will use any nearby objects as high velocity projectiles to kill at greater distances. She is also capable of stopping or deflecting most standard ammunition when she concentrates on the task.
Lucy hates humans mainly because of how she was alienated by her human peers as a child, several of whom she eventually killed after they captured a puppy Lucy had been caring for and beat it to death in front of her. Consequently, she discounts non-diclonii, claiming they are not real people, to the point of telling Nana that she has "not killed anyone yet". She seems to lack empathy and kills without much concern as a usual first reaction. She is incredibly sadistic, amputating limbs and blinding foes, leaving them to bleed to death rather than killing them outright. Despite this lack of concern for human life she will not harm Kohta, and cries and apologizes for killing his family in a fit of jealousy; she loves Kohta, but due to her vengeful nature, she believes he will be content without her existence. As such, she refrains from killing in his presence, except in the anime, when at the end, she kills an entire SAT unit in front of him. However, Lucy still isn't above acts of jealously, once using her vectors to shove Yuka when she saw her holding hands with Kohta. She never refers to herself as Lucy, but her real name is "Kaede", which is revealed when she uses it to sign her letter to Kohta in the manga. After her death in the manga, Kohta and his daughter are approached by two twin girls with ribbons in their hair, heavily implied to have some connection to Lucy and Nyu.
Nyuu 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 "Nyū" 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 be in appearance like his deceased little sister Kanae, whom Kohta claimed to have said horrible things to her before she died in the hands of Lucy. When Kohta mused if Kanae would ever forgive him for what he said, Nyu copied Kanae's appearance of short hair and a bow and, on account of Kohta asking Nyu what she was up to, said, 'I'm Kanae!' and said she forgave Kohta so he wouldn't be sad. 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 (which in flashbacks, was what urged Lucy to start killing), 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.
Informations Source: Wikipedia
