OUT OF CHARACTER
Backtagging: Yes, please! My timezone (GMT) can make things difficult.
Threadjacking/threadhopping: As long as you're not butting into anything important or pre-planned. Use your own discretion.
Fourthwalling: Sure, I don't mind it.
ACTION
Fighting: Yep. Aqua is all about the fighting. She's a Keyblade Master!
Injuries: They happen and I'm for it happening, but Aqua is very good with healing and protective magic, so you'd have to be very skilled to wound her badly. Don't godmod or powerplay, and talk it out with me beforehand if the injuries are serious/life-threatening.
Killing: See injuries. I'm not willing to kill Aqua except for plot purposes, and I wouldn't want to off her permanently.
Telepathy/mind-reading: She'd have very strong mental defences, particularly compared to... well, Terra. She's very strong-willed and would not take kindly to people poking around in her head, but it could happen. Talk it out with me if you're not sure.
RELATIONS
Hugging: Yes! Aqua would be slightly awkward, but she'd return the hug.
Flirting: She'd either be oblivious or very embarrassed, depending on who you are. Either reactions are fine with me. XD
Kissing and/or other intimate actions: Yes, with sufficient CR to support it.
REVERSE PERMISSIONS
Aqua has the ability to sense strong light or darkness -- generally good or evil natures, but not always -- in those she meets. If your character has a strong heart and possesses either strong light or darkness (or even a balance of both, in rare cases like Riku), please comment here and let me know! Strong light is generally reserved for very, very pure people, like Keyblade Wielders and Princesses of Heart. On the flipside, characters with deep darkness in their hearts would generally be evil, or possess very dark thoughts/emotions.
In most cases, your character may not have a remarkable heart at all, and Aqua won't sense anything out of the ordinary. You can also comment here and let me know that, but I'll assume Aqua doesn't sense anything by default so as not to step on any toes.