Lucy is a Diclonius, a human with two cat-ear-like horns with a telekinetic ability called 'vectors'. Lucy is the original Diclonius. Born from humans, she can reproduce in a natural way with humans, just as she can reproduce by passing the virus with her vectors. She is different from every other Diclonius because she didn't hate humans from the beginning. She started hating them when everybody picked on her, specially Tomo, who killed a puppy that she was taking care of just to see her cry. In that moment, Lucy transformed into an assassin. She is a cruel, cold-blooded killer that wouldn't even hesitate to kill a human child. She does, however, like animals.

She was caught by a member of a secret research institute. Eight years later, she escapes but when she is about to succeed, a bullet strikes her head and she ends up losing conscience. She then washed up on shore and was found by Kouta and Yuuka. They took her to Kouta's house to heel her wound, but this is no longer Lucy. Besides losing her memory, she also now has a completely different personalty. Because all she can say is 'Nyu', that's what they name her. She does, however, revert back to Lucy when she is stuck in the head or in danger.

As Nyu, she is sweet and gentle, always trying to make others happy. But because of the blow to the head, all she can say is 'Nyu', hence the name 'Nyu'

Side Note: Lucy is the much smarter version of the two personalties.