Actually, she does have an origin, but it's pretty silly, IMO. She's from 50,000 years in the past, is an Atlantean, grand-daughter of Arion the Sorcerer. She was put in a dimension where time moves much slower to save her from being possessed by an evil being. Her memories of Krypton were implanted, and have since been proved false. All this is from the 1990 issue of Who's Who in The DC Universe, and I don't know if they've ret-conned her back story since.