Tonight's episode of The Flash introduced Doctor Light, a metahuman from Earth-2, as the most recent import from the multiverse bent on breaking Barry Allen's face.
...So, who is she?
Without getting too spoilery, the TV answer is strange and complex. But it also doesn't tell you a lot about the character. That's what the source material is for, of course.
In the comics, there's more than one Doctor Light.
The original Doctor Light was a villain -- Dr. Arthur Light -- who used light-based weaponry, photokinesis and his genius-level intellect to become a persistent (if generally fairly minor) threat over the years. Eventually, it was revealed that he had assaulted and raped Sue Dibny, the wife of the Enlongated Man, when he once found her alone on the Justice League satellite. This precipitated many of the events of Identity Crisis and the lead-up to Infinite Crisis. After that revelation, Light was generally rejected by the readers as irredeemable and on the rare occasion he made appearances, he was almost always reduced to a slobbering, misogynistic pervert. Eventually, he was killed by The Spectre.
Kimiyo Hoshi, the Doctor Light upon which the TV version seems largely based, was a hero and a longtime member of the Justice League. First appearing in Crisis on Infinite Earths, she would take up with the League during the Giffen/DeMatteis era and make such an impression that she would periodically be called back. She had a number of notable adventures, and took part in World War III -- a battle against the forces of Black Adam during the weekly comic book event 52.
It was generally assumed by fans that a reference to "Dr. Light's" gun on Arrow -- used by Cisco against a Mirakuru soldier -- referred to Arthur Light. In the post-Flashpoint DC Universe (the current continuity), Arthur Light was an A.R.G.U.S. agent who died during a freak Superman-related accident, and never a villain.