Every DC Comics fan, or close to it, knows what happened to the original, pre-Crisis Supergirl. In the 1984 "Crisis on Infinite Earths" maxiseries, the Girl of Steel eats a (metaphorical) bullet to save reality from the Anti-Monitor and help clean up continuity for the John Byrne Superman relaunch.
Barring an out of continuity holiday story, and some recent fun in Peter David's "Supergirl" series (which stars another Supergirl entirely), the original supergirl, Kara Zor-El, was gone.
But not without a fight: Current DC Comics chief Paul Levitz was Paul Levitz, "Legion of Superheroes" writer back then. Supergirl had been a sometime cast member of the series -- her longest and most steady (although that's too strong of a word) romantic relationship was with LSH member Brainiac 5, interestingly -- and Levitz wanted to bring the character back after her dramatic death, as commemorated in the classic cover of Superman holding a battered Kara in his arms, weeping openly.
It began when he and artist Steve Lightle decided to create some new members of the Legion to shake up the cast. Lightle posted the story on the official DC Comics Legion board earlier this year:
"Awhile back Paul Levitz and I created a few Legionnaires, and assorted supporting characters. Sensor Girl was always more Paul's baby, just as Tellus and Quislet were more mine. He had this idea of doing a female character that had exaggerated sensory powers.
"One thing that was important to me as I designed this new female Legionnaire character, was that she not be wearing a skimpy, or sexually provocative, costume. There were many Legionnaires that already represented that type. Since I've always believed that diversity was one of the great natural attributes of the Legion, I wanted at least one female Legionnaire who didn't seem to trade on her physical beauty. Sensor Girl wasn't supposed to even have the skintight costume that most heroes wear. You will notice that there have always been wrinkles in her costume rather than the 'painted on' look.
"I thought that there might be an interesting irony in having a character with sensory abilities which was completely blocked from sight, touch, taste, etc. In my original sketches of the character, Sensor Girl's mask was completely featureless, but I decided that taking such an extreme approach was visually boring and unoriginal. After all, Steve Ditko had already created his character The Question with a featureless mask. I then decided to add the color and pupil-less eye shapes, actually they are meant to be stylized feminine lashes. When Paul saw that I had designed a Legionnaire with a full mask, I think that sparked his desire to spare Supergirl from her Crisis fate, by making her the lady behind the mask. I believe the original idea was that even she didn't know her true identity, and that she had lost all her Kryptonian powers except those pertaining to her senses. This is why, in the early appearances of the character, she appears to have powers consistent with X-ray vision, heat vision, etc. It wasn't until [DC Comics president Jeanette] Khan insisted that Supergirl should remain dead (she was very firm on this at the time) that Paul came up with Plan B ... [absent former Legion member] Projectra. after all, who wanted to incur the wrath of Jeanette?"
The mystery as to the identity of Sensor Girl was an ongoing plot line in the late 1980s, with many readers, and Brainiac 5 becoming convinced that she was Kara Zor-El.
"The Kara hints BECAME a red herring, but it was our intention, at the outset, to have Kara in the Sensor Girl costume.
"Paul Levitz is very adaptable. It's a pity I can't seem to lure him back to writing comics. The guy needs extra hours in the day."