Satan Girl, seen below, appeared at a time when all of the female members of the Legion had become ill with a "Crimson Virus" and had to be sent off to Quarantime Planet for treatment.
She offered her services to the Legion as a replacement for the missing members, stating that her powers made her as valuable as all of the lady Legionnaires combined.
Of course, the boys were having none of it, as they were convinced that their absent members would be cured and would return to active duty soon.
Angry at being rejected, Satan Girl insisted that the girls would never recover, and she would know: she was the one who had infected them with the virus.
Satan Girl then headed off to Quarantine Planet to finish of the ladies. Fortunately, Supergirl, who was not affected by the virus, swooped in to fend her off. However, she wasn't able to defeat Satan Girl, whose powers were a match for her own.
It turned out that Satan Girl was actually an evil duplicate of Supergirl, the result of the Maid of Might being exposed to Red Kryptonite, a substance which has unpredictable effects on Kryptonians. However, the effects of exposure only last for 48 hours. Knowing this, Satan Girl realized that her time was short and, wanting to live her own life separate from Supergirl, found a way to extend her existence by transferring the Red Kryptonite radiation in her body into the ladies of the Legion.
(Part of the effect of the exposure to Red K had resulted in Supergirl losing all memory of the exposure.)
Unfortunately for her, the intervention - *sigh* - Legion of Super-Pets prevented Satan Girl from fully realizing her scheme (animals were not affected by the Red K radiation that she emitted to cause the "Crimson Virus"), and as her time ran out she was absorbed back into Supergirl and ceased to exist as a separate being.
I don't think it's really fair to call her an "evil" duplicate of Supergirl. Sure, she lacked Supergirl's self-sacrificing and heroic nature, but can you really blame her for just wanting to live?