Ascended Fanon: It was speculated among Kingdom Come fans that a hero called "Brainiac's Daughter" was daughter of "old-time Legion of Super-Heroes' lovebirds Supergirl and Brainiac 5". Artist Alex Ross has admitted he did not think of an origin as designing the character, but he likes that fantheory, so it is now canon.
Supergirl has been Sterling Gates' favorite super-hero since he read Crisis on Infinite Earths when he was a kid.
Keith Giffen, who wrote and illustrated the Legion of Super-Heroes during the book's most acclaimed and best-selling eras, considers Supergirl one of his favorite Legionnaires, to the point he created an Expy called Laurel Gand, "Andromeda", when he was forbidden from using Kara Zor-El by editorial.
Although most of creatives and fans feel positively inclined towards Kara Zor-El, Marv Wolfman and John Byrne resent her existence because they feel she "de-uniques Superman" (Byrne's own words) by taking his "last son of Krypton" status away from him, and both writers have gone out of their way to erase her from canon. Wolfman famously killed her off in Crisis on Infinite Earths, and Byrne created a genderless artificial lifeform who called itself "Supergirl" so that DC didn't feel tempted to bring Kara Zor-El back to not lose the trademark.
Notorious Legion of Super-Heroes writer Keith Giffen is their polar opposite. It is well known he is very fond of Kara Zor-El, treating her respectfully and going so far to reboot the Legion's continuity and create Andromeda when he was not allowed to use Kara. Every single time he has to write other Supergirls, though, they are depicted as loons or laughingstocks. Case in point with Giffen's feelings? He had Post-Crisis Linda Danvers literally Put on a Bus to Hell and depicted as a Fallen Angel in Reign in Hell. And in the Convergence issues featuring Matrix, she was called a "protoplasmic matrix that thinks it's Supergirl".
Exiled from Continuity: So that Superman was again the "Last Child of Krypton", Kara was removed from continuity for eighteen years.
Supergirl (2005) issues #60-67 have never been reprinted. DC intended to release a trade collecting the book's final issues, but it was cancelled due to the Flashpoint reboot.
Peter David's Supergirl run has never been collected in full. DC reprinted the first nine issues in a single trade in 1998 and the "Many Happy Returns" arc in 2004, and nothing else until 2016, when DC started collecting the series. However, awful sales got the series cancelled before reaching the halfway point of the run.
Newbie Boom:Supergirl (2015) introduced Kara Zor-El to many people who previously knew nothing about her but quickly became a new generation of fans.
Official Fan-Submitted Content: In Adventure Comics issue #397 (1970), the editors invited readers to design new versions of Supergirl's costume. Several of them would be featured in the next issues until issue #410 finally featured the winner costume, which Supergirl would wear until 1983.
Outlived Its Creator: Supergirl was created by Otto Binder (1911-1974) and Al Plastino (1921-2013). They contributed stories from 1959 to 1968. Other writers and artists continued writing Supergirl stories afterwards, and their deaths had no effect on the character or her popularity.
Running the Asylum: Some fans have speculated Kara Zor-El was brought back because several of the primary architects of the Modern DCU are fans of the Silver and Bronze Ages. Whatever the case may be, Dan Didio isn't really a Silver Age fan, and he okayed her return because he felt that Supergirl's backstory had become too confusing — four different Supergirls with increasingly convoluted origins since the death of the original character in 1985 — and a simplified version was needed.
Writer Nick Spencer had big plans for Supergirl before he left partway through his first issue. He was planning on setting Kara up as a leader like her cousin Kal and would have led to the creation of a new Young Justice. The story plans go that the villain would have been the Luthor-Brainiac clone and, to deal with it, Kara would gather a team comprised of Static, Blue Beetle, the Damian Wayne Robin, the Stephanie Brown Batgirl, Miss Martian and the Iris West Impulse (with hopes to include Aqualad). The pinnacle of Kara's evolution would have had Iris running away in a panic and Kara stopping her, convincing her to keep going. Most of the team, minus Kara, would have ended up getting captured and, with advice from Superman, Batman and Wonder Woman, would have made a second team to rescue them. The story would have concluded with the dance party of the Flyover app's premiere, with one scene showing Iris dragging Damian out onto the dance floor. The Jaime/Damian/M'gann team (but none of the others) made it into James Peaty's run, but the details of the storyline were completely altered, along with Alex's true identity ( Kryptonian-Dubbilex hybrid clone).
One of the early issues of the New 52Supergirl series was supposed to introduce a radically redesigned version of Maxima, with her appearance changed from that of a Human Alien to a Humanoid Alien with Pointy Ears. This was scrapped at the last minute and the design was instead recycled for a Suspiciously Similar Substitute named Reign. When Maxima was finally reintroduced in the New 52 several years later, she sported a slightly younger version of her classic appearance.
Anj: You sowed the seeds of future plotlines which may never come to fruition. Can you tell us what you were planning with Blaze? Siobhan? Michael? Shay Veritas? TONY: I was going to have Blaze create a trio of henchgirls called the Furies. Remember Siobhan’s roommate and her two friends? They were all orphaned in superhero battles and had sworn themselves to finding a way to make super-people pay. They were going to beat Supergirl and take her back to Blaze’s home dimension. Hilarity would ensue. I wanted to get Siobhan in there more, to make her and Kara a team. I had one issue where the newly Red Lanterned Kara fights Silver Banshee, and I contrasted their fight with flashbacks of Kara and Siobhan just having fun roommate moments. That remains one of my favorite things from the run: just letting her have a friend. Michael returns in my final issue, and we’ll see if they do anything with him after that. If not, it was enough that Kara met a guy who had every reason to hate life and yet retained a positive outlook. He was in many ways a good role model for her, and he didn’t want to trick or use her, which is nice for a change. And Shay I wanted to keep playing sort of mysterious. Is she really a friend to Supergirl, or does she view her as another science project? Shay’s still a bit of a cypher to me, which is okay if she’s played for mystery. But I liked doing scenes of her staff at the Block talking about what it’s like being stuck there, having to wear her face, alternating between admiration and resentment. It gave the whole Block set-up a little complexity.
Geoff Johns had wanted to include Supergirl in the "One Year Later" Teen Titans roster, but since she was going a Darker and Edgier route at the time and Johns wanted a " naïve, fun alien chick", Miss Martian was created as a substitute.
In "Alan Moore's Writing for Comics!" essay, Alan Moore revealed For the Man Who Has Everything was supposed to feature Supergirl in Wonder Woman's role, but he wasn't allowed to use the former due to her impending death: "...I originally wanted to use Supergirl but then Julie Schwartz informed me that she'd be popping her bright red clogs during Crisis on Infinite Earths and suggested that I use Wonder Woman instead."
In Superman: The Animated SeriesPaul Dini and Bruce Timm wanted to use the original Supergirl, Kara Zor-El, instead of the then-current Supergirl (Matrix/Linda Danvers in the comics), but they were denied by DC Comics, due to the then-current "Superman is the only survivor of Krypton" edict.
There was talk of a Justice League-style cartoon way back in the late 90's. The team would have included Supergirl.
Legion of Super Heroes (2006): According to James Tucker in an interview with World's Finest Online, had the show continued with a third season, Supergirl would be introduced.
Young Justice (2010): Christopher Jones has stated that had the show been renewed for a third season, Supergirl would have appeared. Given that the show has been Un-Cancelled, it could be possible for her to make an appearance and indeed, she finally appeared in the fourth season finale.
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