Lucy

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.

[Spoiler] As a child, she bonded with a dog that was killed right in front of her by classmates, triggering her first killings. She loses all sense of empathy, killing without much concern and acting somewhat sadistic, often torturing victims before killing them (such as Bandou and Nana, though she failed in killing them both). Towards the end of the series, however, she begins to show remorse for her actions, mainly towards Kohta, who lost his father and sister to her.

Kohta was Lucy's childhood friend, though she didn't trust him when they first met. As they became friends, Lucy starts to have feelings towards him. She kills his father and sister out of jealousy when he goes to a festival with his cousin Yuka, which causes him immense mental trauma that makes him forget about her and his cousin. [/Spoiler]

[Spoiler] 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. [/Spoiler]

[Spoiler] At the end of the manga, it's revealed that her real name is Kaede. Coincidentally, Kaede was also the name of the inn Kohta lived in, "Kaede/Maple Inn." [/Spoiler]