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 by the name appeared as an enemy of Spy Smasher.
The first Dark Angel was created by Charles Sultan, first appearing in Spy Smasher #1 (1940). The Wonder Woman villain was created by John Byrne and first appeared in Wonder Woman (Volume 2) #131 (1998)