Dark Angel is the arch nemesis of Donna Troy - and also a version of her from another universe. She was an agent of the Anti-Monitor with a long history of plots against Wonder Woman and was responsible for much of the convoluted history of Wonder Girl.
An earlier character using the same name appeared as an enemy of Spy Smasher. Another version of the Dark Angel was introduced in the Post-Crisis Multiverse as a Supergirl villain.
The first Dark Angel was created by M.W. Wellman and Charles Sultan, first appearing in Spy Smasher #1 (1941). The Donna Troy's doppelganger version was created by John Byrne and first appeared in Wonder Woman (Volume 2) #131 (1998).