Backtagging: I ♥ backtagging, I'll do it quite a bit myself. Let's be backtag buddies! If you think I've forgotten a thread that you want to continue please just shoot me a ping reminding me about it, I have the memory of a goldfish.
Threadhopping: In general I am okay with this, though if you're getting into a 100+ comment long tangent with someone else on one of my posts I'd prefer you move it to a different post? Additionally, if it looks serious or PLOTTY, I'd appreciate being asked first (I will likely almost always say yes). PM via DW or Plurk are both fine!
Fourthwalling: Demyx probably won't believe you, but as long as it doesn't hit INFOMODDING level I'm fine with it? (i.e., an offhand comment about "That damn boss battle" is ok, but please no "Let me tell you your life story")
Offensive subjects (elaborate): I am pretty good to go for just about anything and have been labeled 'unsquickable'. I'd prefer you keep a lid on slurs and hateful language unless you have serious case for it being IC because I just generally find that to be pretty douchey.
Hugging this character: OK - Demyx might even hug back (after a moment of surprise).
Kissing this character: Sure, though he'll see it as a joke and might even play along.
Flirting with this character: Also fair game, though again h is assumption will be that it's just an 'all in good fun'.
Fighting with this character: I'd prefer to plan things out ahead of time so we're both on the same page, but I do love fight scenes.
Injuring this character (include limits and severity): Surface flesh wounds are alright (cuts, bruises, etc) anything further and I'd like to be asked first though I will probably okay it.
Killing this character: Ask first, obviously. I'm not opposed to it if the plot is good, but if it's just to be another body for Mr. Biggest Badass On The Playground I'll probably forego.
Using telepathy/mind reading abilities on this character: A-OK, just ask me first or let me know so I can let you know what you're getting. Demyx has zero in the mental defense abilities, though he is good about saying one thing and thinking another.