Who is Imra Ardeen? She is one of the founding members of the Legion of Super-Heroes; goes by the name Saturn Girl because she comes from Titan, one of the moon's of Saturn; and she predates Supergirl by 4 months. Saturn Girl, along with Cosmic Boy and Lightning Boy, made their first appearance in the pages of Adventure Comics #247 in April 1958. Supergirl made her famous debut in Superman #123 in August of the same year. Both characters were created by the team of Otto Binder and Al Plastino.
Like many characters in the Silver Age, Saturn Girl's powers were quite astounding. She could summon people across great distances, read the minds of humans, animals, and even electronics, and control people's thoughts, emotions, and in some cases, actions. Later they scaled it back a bit to varying degrees, but mostly straightforward telepathy, allowing her to read minds and in some cases control thoughts or actions on the weak-minded. As part of the Legion, she also has her flight ring, which allows her to fly and protects her from the vacuum of space.
The character's history is quite convoluted in that the Legion has been rebooted more times than the standard DCU. As a founding member, she became one of the early leaders and was the first female character to lead a super team in comics. During the Silver Age, she led the team for multiple terms, but did so in an attempt to prevent the death of a teammate, trying to make sure she would be the one. Lightning Lad sacrificed himself instead, as he was in love with her. Later, with a chance to bring Lightning Lad back, Saturn Girl was willing to sacrifice herself again — only to have Proty take her place. With Lightning Lad back, the two were eventually married and retired for a brief moment only to be called back to active duty when the active members were all captured.
Reboots after Zero-Hour, Infinite Crisis, something called Threeboot and DC Rebirth have all made the Legion's history very convoluted. She appears in the recent Button story where she is seen screaming about the death of the Legion while watching a hockey player die on television.
Saturn Girl has mostly been portrayed as a human-looking blond girl with a leaner build. The Supergirl version has her with dark hair. She has appeared in Superman: The Animated Series voiced by Melissa Joan Hart, in Justice League Unlimited and of course in the Legion of Super-Heroes animated series voiced by Kari Wahlgren. She appeared once before in live action played by Alexz Johnson on an episode of Smallville.
Imra Ardeen is being played now by Amy Jackson in Supergirl and made her first appearance in the seventh episode of the season. She was unconscious through most of the episode, but woke up at the very end when her name was revealed, along with her being Mon-El's wife. She did not appear in the Crisis on Earth-X crossover episode, so tonight's mid-season finale will be the first full appearance of the character.