Kaede, who is known as both Lucy and Nyu, is a Diclonius and the central figure of the narrative. A Diclonius is a mutated human subspecies that possesses two horn-like protrusions on the head and invisible telekinetic limbs called vectors. Kaede was born from human parents but is the original Diclonius, sometimes referred to as the queen of her kind. As a child, she was subjected to severe abuse and ostracization by her human peers, who mocked her horns and called her a freak. A pivotal event in her early life occurred when a group of classmates killed a puppy she had been caring for, beating it to death in front of her. In response, she brutally murdered them, an act that marked the beginning of her violent tendencies and deep-seated hatred toward humanity.

Following this incident, she was captured and held in a research facility. Eight years later, she escaped, leaving a trail of destruction. During her escape, a large-caliber bullet pierced the metal helmet encasing her head, causing trauma that resulted in the formation of a second, childlike personality called Nyu. In this state, she washed ashore near Kamakura and was found by Kouta and his cousin Yuka, who took her in and named her Nyu after the only sound she could initially make.

Lucy and Nyu represent two distinct personalities within the same body. Lucy is cold, ruthless, and sadistic, killing without hesitation and often torturing her victims. She views most humans as subhuman and shows little empathy. She is driven by a deep hatred born from years of mistreatment and betrayal. However, she is not entirely devoid of positive emotion. She loves Kouta, whom she met as a child, and feels immense guilt over killing his father and younger sister in a fit of jealousy when he went to a festival with Yuka. Her primary motivation throughout the story is twofold: to avoid harming Kouta further and to seek some form of redemption or reconciliation with him. She often deliberately lets the Nyu personality take over when Kouta is present to shield him from her violent nature. Nyu, by contrast, is innocent, gentle, and has an infantile understanding of the world. She cannot speak beyond the word nyu for much of the series, though she gradually learns more words. She is incapable of violence and forms a strong affectionate bond with Kouta, whom she trusts completely. Nyu is not simply a separate creation of trauma; Lucy subconsciously encourages her presence as a way to hide from her own guilt and to prevent herself from harming Kouta again.

As the central protagonist, Lucy/Nyu drives the conflict of the story. Her escape from the facility sets the plot in motion, and her dual nature creates the central tension between violence and innocence, vengeance and love. The narrative follows the attempts of the research facility and a special assault team to recapture or kill her, as well as the efforts of other Diclonius sent to eliminate her. At the same time, the story explores her relationship with Kouta, who is initially unaware that Nyu is the same person who murdered his family eight years earlier.

Lucy's most significant relationship is with Kouta. Their childhood friendship was the one bright spot in her otherwise cruel upbringing, and he was the only human she trusted. When she killed his family out of jealousy, it shattered any possibility of a normal life between them. Despite this, she continues to love him and desperately wants his forgiveness. Kouta, for his part, suffers from trauma-induced amnesia regarding the event and slowly pieces together the truth as the story progresses. Their eventual confrontation is the emotional climax of the narrative. Other important relationships include her antagonistic interactions with Nana, another Diclonius who is initially sent to hunt her but later becomes a more sympathetic figure. Lucy also has a complex dynamic with the scientist Kurama, who is responsible for containing her. She shows occasional jealousy toward Yuka, who is close to Kouta, but does not kill her, highlighting a restraint that contradicts her otherwise murderous nature.

Across the course of the story, both personalities undergo development. Lucy begins as a near-unstoppable force of vengeance but gradually reveals deeper layers of remorse and longing for connection. Her hatred is shown to be a result of her treatment rather than an inherent trait, and she demonstrates the capacity to care, particularly for animals and for Kouta. Nyu, meanwhile, evolves from a purely helpless and dependent persona into one that can, under certain circumstances, act with greater awareness and even use her Diclonius powers, though typically without the same lethal intent. In the later parts of the manga, a third instinctual personality is also suggested, one that embodies the pure Diclonius drive to exterminate humanity.

Lucy possesses vectors, invisible arms of telekinetic energy that she can manifest from her back. She has six vectors, each with a typical range of about two meters. Within this range, she can slice through flesh and solid objects, lift heavy items, and deflect bullets. She can also accelerate objects, such as stones or debris, to high speeds, allowing her to kill targets well beyond her vector range. Damage to her horns can impair or nullify her abilities, and if both horns are lost, she may become comatose or die. In the manga, her vector range and power increase significantly after her horns regrow, extending far enough to reach a helicopter in flight. Her abilities make her one of the most dangerous Diclonius, capable of single-handedly defeating armed military units. Despite her power, she is not invincible and can be subdued by surprise attacks or by exploiting her emotional vulnerability, particularly her feelings for Kouta.