A live-action Supergirl TV series is in development for the CBS network, which is interesting, because her comics are a mixed bag. Rather tellingly, DC Comics never printed a Greatest Supergirl Stories Ever Told reprint collection, but they have with Batgirl, Green Lantern, even Captain Marvel (!), who can only be reffered to as Shazzam nowadays because Marvel Comics wouldn't like that. For a long time ( and I'm making a long story short ), Supergirl couldn't even be Superman's cousin! Oh, she could be the shape-shifting clone of Lana Lang who assumed the identity of an insane goth girl named Linda Danvers and later develop new powers and complications that made her "Supergirl" in name only that were relevant in the wake of pop culture trends like The X-Files and Buffy, The Vampire Slayer...but it wasn't Supergirl.
Honestly, I predict a Supergirl TV show will ignore the comics for the most part, which I believe is the formula that's made recent series like Arrow and The Flash successful - both have very little in common with the source material at all, even Gotham, which claims to draw inspiration from bits and bobs of Batman comics, but is really evolving into a Game of Thrones-style series about the origin of Batman, his supporting cast and rogues gallery as they converge in Gotham City. And Batman is in it - as young Bruce Wayne, who is portrayed as an armchair detective of sorts. Again, there are no callbacks to any specific comics; anyone who believes so is trying to reach hard.
So I doubt Supergirl will be based on any comics. Her characterization will probably depend on whoever is cast in the role, too. A fanpage devoted to the character on Facebook characterized her as a "warrior princess"...where did THAT come from? As for villains? They could lean on the Superman rogues gallery for villains least-likely to appear in the films, like Livewire, The Prankster, Mr. Mxyzptlk, etc..and Reactron - the only rogue I remember from her comics. Ambush Bug might show up (because he appeared in her comic once), possibly Granny Goodness and the Furies will be big players...maybe Kara will take up roller-hockey or join a rock band, becoming a Riot Girls-esque hipster - who knows? Maybe Batgirl will show up to set up a spin-off series?
If I had to imagine a premise for this character...I would say that instead of arriving on Earth after the destruction of Krypton, her rocketship/escape pod landed on the future Earth of The Legion of Superheroes and...having spent a brief, yet significant time as a member of the Legion, time travels to present-day Earth to check up on Superman and perhaps save the Earth, but ultimately decides to stay. So...we've got a Supergirl who was a ring-bearing member of the Legion (and also a former member of the Red Lanterns) but is new to Earth. That's a sketchy outline for a pilot, but I see this character as someone who is.. a nice, but relatively aloof foreign exchange student arriving in a new place.
Meanwhile, there's cool art from fans, professional fan-favorite artists and cosplayers on the internet..I particularly like model/cosplayer Enji Night's Supergirl and the cape-hoodie combination design on one illustration...but then, I selected all of these pictures for this post, so of course I like them all! Enjoy!
Update: more details about the show have been revealed - Supergirl's civilian identity will be "Kara Danvers" and she'll have an older step-sister named Alex. They're looking for actresses between the ages of 22-26 to play 24 year-old Kara. After glancing at other websites offering suggestions, I like the idea of casting Claire Holt as Supergirl/Kara and I would suggest Alona Tal as Alex..of course, they could just go with casting unknown newcomers in both roles, as they did with The Flash and Green Arrow. The wheels are in motion in Hollywoodland..
UPDATE: Melissa Benoist has been cast as Supergirl. The magic of fan art allows me to imagine a series that might appeal to fans of "Veronica Mars" and most of CBS' prime time lineup of detective procedurals. I suppose it'll be fun to see Supergirl hold a magnifying glass with her heat vision to give the bad guys a hot foot... :)