Lucy is a Diclonius girl around eighteen years old. She has developed strong emotions of hatred and vengeance towards regular humans mainly because of how she was treated by the majority of them as a child, making fun of her horns and giving her insulting nicknames such as freak.
As a child, she bonded with a dog which was killed right in front of her by classmates, which triggered her first killing. She seems to lack empathy, kills without much concern, and acts somewhat sadistically, often torturing victims before killing (such as Bandou and Nana, though she failed in killing them both); however, towards the end of the series, she begins to show remorse for her actions, especially towards Kohta, whose father and sister were killed by her. Kohta was Lucy's childhood friend, she didn't trust him much in the beggining but later starts to have feelings towards him. She killed his sister and dad after Kohta lie about the festival he was going, saying that he'd be there with a boy; instead, he went with Yuka, his cousin.
She is aware of her "Nyu" state as revealed in the manga by her worries just before she reverts. "Lucy" is not actually her real name, but the name of her inner Diclonius killing instinct that wants to exterminate mankind, her real name is "Kaede".