The character known as Lucy or Nyu is the main protagonist of Elfen Lied and belongs to a mutated subspecies called Diclonius. Her original name is Kaede, and she is later designated as the Queen Diclonius due to her unique abilities and significance to her species. As a child, she was subjected to extreme cruelty and isolation from humans, who feared and abused her. The turning point came when a group of children captured a puppy she had befriended and beat it to death in front of her, prompting her to retaliate by murdering them in a brutal manner. This event cemented her deep-seated hatred of humanity, which she often expresses by dismissing humans as not real people and killing them without remorse. She displays a sadistic tendency in combat, severing limbs and leaving victims to die slowly.

Lucy possesses telekinetic appendages known as vectors, invisible arms that can extend roughly two meters from her body. These vectors can slice, punch, lift, and manipulate objects, and she can also use nearby items as high-velocity projectiles to attack beyond her immediate range. With concentration, she can deflect or stop most conventional gunfire. A secondary function of her vectors is the transmission of a virus to human males through touch, which ensures that any future children they father will be born as Diclonius. She also possesses enhanced senses that allow her to see the otherwise invisible vectors of other Diclonii.

During her escape from a research facility, a bullet pierced her head, resulting in the emergence of a second personality: Nyu. Nyu has a childlike demeanor, minimal language skills (initially only able to say Nyu and later Kouta), and is completely incapable of violence. This alter ego serves as a psychological refuge for Lucy, who deliberately allows Nyu to take over when she feels guilt or wishes to avoid harming Kouta. In turn, Nyu can revert to Lucy if she is attacked or struck on the head. A third personality, representing the raw instinctual drive of the Diclonius queen, occasionally manifests and fuels Lucy’s most destructive impulses.

Lucy’s motivations are driven by her need for connection and her desire to be forgiven by Kouta, the only human she ever loved. She met him as a child and developed genuine affection, but her jealousy and the influence of her darker instincts led her to murder his father and younger sister. She carries an immense burden of guilt and tries to hide her true nature from him, often using Nyu as a mask. Her central goal throughout the story is to apologize to Kouta and, in doing so, find some form of redemption.

Her relationship with Kouta is the emotional core of the narrative. She refuses to harm him and becomes visibly distressed when she thinks of her past actions. She also feels jealousy toward Yuka, Kouta’s cousin, but stops short of killing her. With other Diclonii, such as Nana, she exhibits a degree of kinship, stating that she has not killed one of her own kind. Her interactions with humans outside of Kouta remain hostile and lethal.

Over the course of the series, Lucy shows a capacity for change. She struggles to suppress her violent urges, especially when Kouta is threatened, and in the final confrontation she tearfully apologizes to him. In the anime adaptation, she appears to sacrifice herself after promising never to kill again. In the manga, her development is more prolonged, culminating in a final act of protection that leads to her death or disappearance. Her journey illustrates the tragic consequences of rejection and abuse, as well as the possibility of remorse and the search for acceptance, even in a being designed for destruction.