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Well, that sounds a heckuva lot like the first season of The Legend of Korra...
No, it wasn't Legend of Korra. It didn't have anything to do with an avatar. Thanks anyway!
You mentioned you've looked at lists of 2000's anime, but have you checked the Adult Swim programming pages? There's this: https://en.wikipedia.org/wiki/List_of_programs_broadcast_by_Adult_Swim#Syndicated_programming
and then this for their Toonami block which started in 2012: https://en.wikipedia.org/wiki/List_of_programs_broadcast_by_Toonami#Toonami_lineups_on_Adult_Swim_.282012.E2.80.93present.29
I don't know if this helps, but I believe the main character also wore some kind of goggles (maybe steampunk?) and cargo pants.
@livewithstyle I have looked at those. But, I checked them again now just in case I missed something. Still nothing. Thanks, though!
Was it Oban Star Racers? The main character Eva/Molly is a 15 year old girl, has brown hair, pilots a spaceship and dresses like you describe.
@vpetmad I found the opening for Oban Star Racers on You Tube to see if it could jog my memory. It didn't look exactly like I remember, but perhaps my memory has been distorted in the years since I saw it. I'll check out a few episodes. Thanks for your help!
Last Exile has a steampunk feel and muted color scheme.
Edited by PegasusKnightmare
@Pegasus Knightmare I looked that up and it doesn't ring any bells. Thanks anyway!
It was an anime, I'm almost certain. If it wasn't an anime, it looked a lot like one. It's been several years now since I watched it. So, the best timeline I can come up with is 2007-2011. It had to have come on Adult Swim, because I remember feeling as though I shouldn't be watching it. Not that it was a "bad" show. From what I remember, (and I only watched one or two episodes) it was pretty sfw. The main character was a girl with short-to-medium-length brown hair. She had to of been at least 16 or older. She was kind of tomboyish and I remember there was a scene either in the episode itself or the opening where she was wearing a white sports bra. I remember some sort of arena, perhaps a stage for battle. There was spaceship like aircraft which I'm pretty sure was a main part of the story. They might have used these to battle. I remember a scene in the episode I did watch where the main character was laying in her bed (she, like, lived in the arena building??) and a tall man (I'm guessing he was an adult, or at least older than her) with I believe blonde hair walked in and they argued about something. Perhaps she had done something dangerous and he was scolding her. I do remember the show having a rather dull color scheme. Mostly browns and grays. This is all I can really remember, sorry. After all, it's been quite a while since I saw it. I've already spent several hours trying to find it on my own. Going through lists of "2000s anime" and character databases to no avail. So, any help will be greatly appreciated! Thanks in advance!