Silpelits are secondary Diclonius created either by the Queen Diclonius or by other Silpelits through the infection of human males via their vectors, causing them to sire Silpelit offspring. They make up the overwhelming majority of Diclonius in existence and seen in the series. They age more rapidly than humans, sometimes looking over twice their physical age. The term is one that refers to the offspring of a human who mated with a monster, and is used in the original poem Elfenlied by Eduard Mörike.
While all Diclonii seem to have a drive to kill and infect Humans in order to displace them, the series often makes it difficult to determine if this is caused by an actual DNA-based imperative or the reflected resentment of how these girls were treated (the series neither shows nor speaks of Male Silpelits, given that they all descended from a fertile female Diclonius, although one primary Diclonius was male). Adding to the problem of figuring this is that the age most Silpelits gain their abilities is three, an age all children are known for becoming very willful, stubborn, and quick to anger. Many Silpelits are in fact found after murdering their own families. The number kept in captivity at the Diclonius Research Institute eventually grew so great, project head Chief Kakuzawa ordered his son and Kurama to simply euthanize any more found right after birth. Some of those killed were also harvested for further research. Among those spared for reasons of study and containment were Nana and Mariko, the latter being his own daughter, and the other developing a parental bond with him, further muddying the issue of nature versus nurture. With what little attention Nana was given, her personality is many times less alienated than others of her kind, while Mariko at times behaved monstrously, due in large part to her nearly complete isolation and emotional manipulation at the hands of Saito.
Over a thousand clones of Mariko were bred, the majority growing up under literally hellish conditions and becoming monsters, with even the viable ones exhibiting extreme hatred of all humans when not under control. Mariko was the most powerful Diclonius of all, possibly the result of being born from an infection by a Silpelit one generation removed from Lucy herself. Due to the schemes of the Kakuzawas, potentially a million or more Silpelits were born even after Lucy's death, due to the deliberate explosion of a missile bearing pure samples of the Diclonius virus.
A narrative offered up by Kouta at the end stated that conflict between the remaining Diclonii and humans threatened the latter group's existence until the worldwide dissemination of an Anti-Diclonius Birth Vaccine which prevented further Diclonius births, and enabled the lifting of a birth ban. Its success seems to be indicated by the non-horned birth of Kouta's daughter, Nyuu, ironically named for the being the Silpelits called their queen.