The diclonii, otherwise known as two-horned humans, are mutants of the human species that may well be the next step in human evolution. The diclonii have horns and strong telekinetic powers represented by arms. However, with this great power, they could easily destroy the human race.

Fearful of their power, humans quarantined the diclonii into secret research facilities to study them. However, in a freak accident, an enraged female diclonius escaped, killing many guards in the process. Interesting enough, the female escapee appears to have suffered amnesia after her escape, and floated to Yuigahama of Kamakura city. There, she meets Kouta and Yuka. They name her Nyuu, and decide that they all should live together.

[Source: AniDB]