Diclonius

The Diclonius is a new species born from a form of human mutation and the main focus of the anime/manga franchise Elfen Lied. Though born from humanity, this new breed differs significantly from humans in both biological and cultural respects. Diclonii possess awesome psychokinetic abilities which they use to kill humans and are driven by a natural instinct to destroy humanity and replace them as the dominant sapient race on Earth.

Biology
The differences that separate Diclonii from humans are both internal and external. The most obvious difference is their horns on the parietal section of their skulls. Most often, these horns resemble cat ears from certain angles. These horns are a mixture of bone and cartilage. In the manga, it is mentioned that Diclonii fall into a temporary coma if they lose one of their horns, and would never return from their lethargic state if they lose both horns. However, Lucy's horns grew back twice: within a matter of months for the first time, then over several days the second time.

Another notable difference between them and humans are their hair and eye colours, though this difference varies from manga to anime. In the manga, Diclonii possess hair colours of many different shades, from pink to purple to blonde, and they all possess green eyes. In the anime, however, their hair colours are more unified, being a variety of pink hues. Likewise, their eye colours are variants of red that can range from very deep red to almost pink. This design change from manga to anime was most likely chosen to make them seem more obviously alien from humans.

Inwardly, they differ from humans in relation to their pineal gland. Normally, the pineal body is the size of a pea and located in the centre of the brain between the two hemispheres, but for a Diclonius, it's the size of an egg and wedged against the frontal lobe. The pineal gland appears to be the source of their vectors, since Diclonii have had their vectors disabled in-series by way of Nana prodding the front of their skull with her vectors and by the mind control devices implanted by the Diclonius Research Institute's scientists.These vectors resemble hands, but cannot be seen, touched, or otherwise acted upon by outside forces (aside from instances in the anime where vector-on-vector grappling takes place). Vectors spread a viral infection to humans via touch which causes those infected to sire only Diclonius children. This is how the Diclonii propagate as they cannot breed amongst themselves. Vectors can also manipulate objects, act as shields by decelerating projectiles, and vibrate at such a high frequency that they can cut through virtually any material, giving them the potential to be massively destructive if used offensively. Against humans, they are almost always lethal. Their vectors usually have a limited range of a few meters. Lucy's are able to extend to two meters, and while Nana, Mariko, and her clones are shown to have either more vectors or greater normal range, Lucy's raw power became strong enough to threaten the world. Ultimately, using her vectors to world-destroying capacity compromised her body's integrity until it literally fell apart.

Diclonii have the innate ability to detect one another, such as when Kurama released Nana from the offshore laboratory to search for and retrieve Lucy. Even this ability had limits, as she could not sense her target when she was in her childlike Nyu personality, nor could she sense the Mariko clones when they were under the control restraints placed upon them by Nousou and the Agent during their assault on Maple House. Likewise, Nyu's presence could not be sensed by other Diclonii.

Diclonius are extremely intelligent, however they are just as emotional as humans, often more so. Once they reach the brink of sanity, some hear a voice that claims to be the directing voice of their own DNA, urging them to kill as many humans as possible.

Hierarchy
The Diclonius race possesses two distinctive castes in-series: Queens and Silpelits. The primary difference between these castes is that one can reproduce and the other cannot.

Those that can't breed are called Silpelits. Referred to as "worker bees," they use their vectors to infect humans with the Diclonius genetic virus on and turn these humans' children into Silpelits when they are born. Silpelits age much faster than humans, with their bodies looking twice as old as their actual age, and their vectors tend to be longer than those of the Queen caste.

The Queen caste, of which Lucy is the only known member, can reproduce sexually. Additionally, they age like normal humans and appear to have shorter vector ranges. However, since Lucy is the only known Queen, it's inconclusive if her vectors are short due to her caste or if they're an individual factor.

There are no male Diclonii in the series aside from Lucy's half-brother. Since he was not a Silpelit, was capable of reproduction, and male, it's unclear if he would count as a Queen or another new caste entirely. Following the analogy to bees, a probable name for his caste could be Drone.

Destructive instincts
It is said by some in-story that the Diclonii were chosen by God as the new Humanity, born with the instinct to destroy the human race and repopulate the world. In the final episode of the anime, Lucy tells Kouta that she was born to kill humans, an idea cemented by her third personality born from the supposed instinct to kill humans all Diclonii are said to have. However, Nana's character, as well as the incidents surrounding Lucy’s past and relations with Kouta, suggests that it is possible for Diclonii to have empathy towards humans, and that their sadistic characteristics develop as a result of their interactions with the world. Barbara, a clone of Mariko, claimed to have also heard this voice, but her personality was shaped by birth and life in a lab. Nana once seemed to hear it, but was in shock from leaving Kurama and was taunted by Bando as to her lack of a place in the world.

It may be said that the homicidal tendencies are a result of an unstable psyche, itself a result of congenital recessive mutations, aggravated to such high levels by inhumane treatment from Humans. Through the run of the series, the question is raised to the audience of whether the Diclonii's homicidal tendencies are an inborn drive to eradicate Humans, or if, like any "normal" Human child, they develop this drive in response to years of trauma and abuse. Their homicidal behaviour is only ever directed towards normal humans, as they behave benevolently towards animals. This is possibly exemplified in the anime when a young Diclonius is shown killing a human for laughing at her, but when the human's dog bites her hand, she reacts by crying instead of harming the dog. However, it should be mentioned that the aforementioned anecdote in the anime was only shown in a story told by Professor Kakuzawa, whose trustworthiness is questionable at best.

Humanity's threat
Diclonii tend to be heavily discriminated against (infants are often euthanized on the spot) by other humans, and the series makes it unclear whether the often murderous hostility Diclonii show as well is innate or the result of the hatred and contempt shown against them.

Researchers at both the Kakuzawa facility and the government-controlled facility believed the Diclonii to be a serious threat to the human race and kept them in isolation. When Kurama's daughter Mariko is born as a Diclonius, he remembered an incident in which an escaped Silpelit touched him with her vectors. At this point, he realized that this explained his daughter's condition and the sudden surge of Diclonius births. With Diclonii passing on their DNA to humans, the infected parents have horned children instead of regular humans, which would gradually lead to the Diclonius race replacing humans as the planet's dominant species. Lucy mentions the danger to the human race to Kouta in the final episode, stating she could make sure more Diclonii are born than humans in just five years if she put forth the effort.

Known Diclonii

 * Kaede (also known as Lucy or Nyu)
 * Nana
 * Mariko Kurama
 * Alicia
 * Barbara
 * Cynthia
 * Diana