She's a teenager in a lot of ways regardless of how old she appears to be. Her primary personality trait is a strong stubborn independent streak. She's P-O-W-E-R Girl not S-U-P-E-R Girl and you best remember that. And she only took that name because she doesn't want everyone in the world on a first-name basis with her. And if she manages to save your life, stop acting like she'd some sort of hero- she's not some attention hog whose trying to get her name in the paper or impress some ditzy reporter who can't even see through a paper-thin disguise.
And about that whole secret identity bit, don't expect her to go around pretending to be all meek and mild. No glasses. No wigs. No orphanages. She'll dress like a regular everyday girl. Provided her lame cousin hasn't spread her real name all over the place in his gossip monger job, she'll be Kara Zorel- awesome but totally human girl. And until she finds a way to earn some cash she'll be crashing at the Fortress and borrowing a few bucks from Kal (since he had the luck to arrive here first....). Eventually she'll find some sort of work but not until she trips into a job that she actually enjoys and that makes use of her Kryptonian skills (not the powers but the stuff she knows from her "life" on Krypton).
Now if you manage to catch her off guard you'll realize that a much of the above is a front. As much as she grumbles about Kal, she better not hear you doing the same. She's actually proud of her cousin, even a bit envious of his growing up here. She still wouldn't want to be Clark Kent, who she finds a bit humorous, but if truth be told she'd love to be Supergirl ... if she could somehow do it without being pigeon-holed as Superman-lite in the process. So she's decided to make Power Girl who SHE thinks Supergirl would be if there was no Superman level expectations to live up to.
And as Kara away from all Superman's allies she is much less cocky and much more open. Power Girl will get in your face if you challenge her. Kara will stand up for herself but in the least confrontational manner possible. Power Girl tends to dominate the situation, taking charge and attracting attention. Kara tends to fade into the background unless she's drawn out by the people around her. She's not Clark who actively works to avoid attention, but rather a person who is trying to get a feel for the people around her before joining in.
She's not sure how a girl raised on Krypton can find common ground with kids her "age" in the USA. They're thinking about getting a car to drive around town, she's already able to fly anywhere on Earth. They grew up with the Internet, she grew up with telepathic headbands. So Kara listens to them talk about the latest pop sensation, YouTube viral video, etc and hopes the next thing she says doesn't make her sound like a total idiot.
Her costume, if you must call it that, would be the Kryptonian equivalent of work close. maybe their version of a T-shirt and jeans or the type of thing you'd wear to work on a casual Friday. It may even be that she has a few different costumes which are all just everyday outfits she'd have worn "back home". The only exception being that she won't wear anything that comes too close to Superman's color scheme and that after Kal bastardized the family crest she wants nothing to do with it.
@captainmarvel4ever: Shallow its the reason why she is skinny now, besides the whole fact you try to ignore how bad is the new Squad when you compare it with the older material.
If you love the only thin, attractive thing that is going on in comic, that is your problem, but not all of us like this generic in generic land, she was special by the whole fact she wasnt like the rest and didnt need to be hot to be powerful and great.
Waller was that she was an interesting female character without being oversexualized. This is now gone. You lose the uniqueness and write diversity out of your universe for no reason.
There's a billion sexy people in the DC Universe, Waller doesn't give a fvvv(k, she comes in, takes charge, and says "I'm not here to impress you, I'm here to keep the peace and stop a bunch of overgrown children from destroying everything around them because they take dress-up and playtime too seriously, even if that means enslaving a bunch of other overgrown children. Now get me a meatball sub, and if that thing gets within 12 inches of whole wheat bread, that explosive collar on your neck will detonate so fast, it'll make your head spin.
John Ostrander on Skinny Waller:
"That said, I think the changes made in her appearance are misguided. There were and are reasons why she looked the way she did. I wanted her to seem formidable and visually unlike anyone else out there. Making her young and svelte and sexy loses that. She becomes more like everyone else. She lost part of what made her unique."
Amanda Waller being fat really reinforced the idea of the common man rising up against these gods and heroes. She busted her ass to get where she was regardless of her physical appearance.
Isnt even they made her muscular and big, thing that make sense with a special op soldier, then for the same stop this all women have to be skinny and sexy sexist crap, they made her a skinny model type woman.