OOC

Backtagging: Always and forever.
Threadhopping: Sure!
Fourthwalling/Canon puncture: I'd rather you didn't, but if you think the situation calls for it, ask me!
Offensive subjects (elaborate): Yuffie herself doesn't do anything offensive, but anything involving rape or incest is a flat-out no.

IC

Hugging: She will probably hug back unless you're a creep, then she'll probably hit you.
Kissing: This might get you injured unless you're Cloud she likes you.
Flirting: Sure, though she might be an asshat in return.
Fighting: Go for it! Yuffie's from end-game, though, so unless your character is the equivalent...she could probably do some real damage.
Injuries (include limits and severity): Sure, but I'd like a heads-up first.
Death: Ask first.
Mind reading: Sure, but it'll probably be boring.