Lucy; or her real name Kaede, the original Diclonius and source of all the Siplets.
As a young girl, Lucy had always been excluded and ostracized by her peers for her two cat ear-like horns. As she grew older, she came to realize an instinct inside of her - telling her to kill human beings in order for her species to claim the world. After witnessing a puppy she had been caring for beaten to death by a group of bullies as well as being betrayed by her only friend, she subconsciously kills her peers instantly. While visiting the puppy's grave, she meets Kohta - a boy who accepts her for who she is. She and Kohta build a friendship to a point where Lucy develops feelings for him as well. When Kohta tells Lucy about the gender of her cousin in order to prevent her from being jealous, she goes to find him in a festival, where she learns that his cousin Yuka is actually female. In a blind rage of jealousy, Lucy kills several people at the festival, unbeknown to Kohta. Lucy soon after finds Kohta in a train and murders his family - sparing Kohta.
After a few years of rooming Kamakura killing humans for any food money or clothing necessary she would
find Aiko her second love. Aiko an aspiring painter helped her after some kids threw a ball at her and treated her with kindness, bringing her back to her house where after a series of events Aiko’s abusive father is killed. They would run away together to find Aiko’s mom in an art exhibit. There Chief Kurama would find her and while trying to capture Lucy and shooting Aiko by mistake. Trading her freedom for Aiko’s life Lucy allows herself to be captured in exchange for Aiko being sent to a hospital. In failing to so Lucy begins her vow of vengeance on Kurama claiming that she would never kill him but kill anyone he loved.
After being contained by Kurama in a Diclonius Research center and Suffering horrible experiments. Lucy escapes while being shot in the head by a .50 BMG round. This causes her to develop an innocent split-personality after being found ashore by Kohta and Yuka. There, she is taken in by the two - later alternating between " Nyu" and Lucy at different, unpredictable situations. This is where the core story starts.
Lucy is often shown to have complete disregard for human life - effortlessly slaughtering anyone who she deems useless. However, she cares for Kohta - most likely Due to her love for him and regret for harming him out of jealously.
She is rather calm preferring to slip away if possible instead of making a big scene. Her domineer is rather cold and lonely though quickly changing to anger when frustrated or given an opportunity for revenge. However sadistic she seems to fall for anyone that will treat her with true kindness.
During combat, she is a ruthless sadist, ripping limbs apart and even poking eyes out.
In the manga she differs greatly. Her Vectors are no longer limited to four and as she grows progressively older she grows more powerful and Vectors more numerous. It is arguably believed that Lucy is like her alternate self. This is more heavily supported by the Manga.
In the manga, Nyu learns to speak coherently in which her personality is far more evident as a kind child like polite girl. She is also a bisexual in the manga and eventually gains the ability to use vectors but never to the extent of Lucy.
Lucy in the Manga has multiple forms (Anime has only two) that are only achievable for her and assumingly any other “Queen/King Diclonius.”
Seen in both the anime and Manga Young Lucy or the fan nicknamed Lo-Lucy is Kaede’s young form. At this form she has a very short range of 1.4 meters however her vector strength is far beyond any other Diclonius. At this age however she does touch on the ultimate form.
The main form in both anime and Manga she differs in abilities depending
on media. The anime she has only four vectors and 2 meters of length
however in the manga she has unlimited vectors and a 4 meter length.
This form is the most common. Nyu only ever achieves this form.
Only achieved after a greatly stressful event Long-horned form gives Kaede extremely long sharp horns as well as vector length and strength approximately three times that of regular Kaede. This is exhibited by her bringing down a helicopter mid flight and destroying most of forces as well as Mariko clones in one sweep.
The Ultimate form only achievable after great emotion stress meeting a surge of willpower. This form is only achieved twice. Once when Aiko died and for the last volume when Kaede decides to fix everything she or her kind caused and to apologize to Kouta. This form has vector strength to destroy the world length to reach across the world and accuracy to move individual cells without killing them and the added bonus of controlled explosive burst. However taking this form has a consequence continues use of such advance powers causes the body to overheat and burn away killing the user.