The Dark Elf, also known as the Mother Elf, is the central antagonist of the Megaman Zero series.
She is a powerful and sentient supercomputer who is once the first of Cyber-Elves, elf-like, living computer programs comprised of pure energy.
Before the events of Mega Man Zero, the Mother Elf once created a worldwide conflict. However it was the first of such attempts to actually be successful on a large scale.
After being "saved" from death, Elpizo declares "she is not evil..." and, ironically and possibly symbolically, the Mother Elf changes into the Dark Elf against her will. This theory was very popular at one time.
However it is dispelled by Rockman Zero Official Complete Works (released in 2006), which states that the Mother Elf was originally created from the Maverick Virus data in Zero's body (the reason why he feels that he knows her), and therefore she is not a former Reploid.