I purchased VF5: Final Showdown not too long ago. The legends of online play being ruthless are true. I was getting destroyed. I use Jacky, Vanessa, sometimes Lion and picking up Shun Di. It's funny because I used to use Aoi a lot in VF4, but I don't really touch her in this game. I'm not patient enough to remember all her reversals.
Congrats on your purchase and welcome-back to VF!
Hmm....Generally, you may want to focus on one character to begin with..Given that there is so much to learning even one character entirely...Not to mention, trying to get comfortable with all of the system mechanics as well.
That said, you seem to have prior experience with Aoi and may be able to be somewhat comfortable with her starting out...As far as what you were saying about her reversals, that's just a matter of time and getting used to the game's speed/characters' moves...Seriously, this game is fast!
After a while, you'll have enough experience and your reactions will be at the point where you'll know when to easily counter attacks with her reversals on the fly without having to break your flow too much.
At any rate, if you like her-stick with her!
Here's some Aoi things that you can check out to get you going though if you want:
VF5FS System Portal:
http://virtuafighter.com/wiki/index/
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Aoi:
http://virtuafighter.com/wiki/aoi/
Aoi Command List:
http://virtuafighter.com/commands/index.php?ver=5fsa&chara=aoi
Aoi Combos:
http://virtuafighter.com/combos/index.php
(^Edit: Sorry! Just go up to the drop down box and choose Aoi! ^^; )
Aoi Character Forum:
http://virtuafighter.com/forums/aoi.76/
Or for whoever you choose to main, the same things can be found on VFDC(and it's wiki) for that character! Good luck-and I'd be more than happy to help you out in any way!