Backtagging: Always happy to!
Threadhopping: I don't mind at all.
Fourthwalling: I'm open to it, but please ask first.
Offensive subjects: None that I can think of.
Hugging this character: It really depends on the who, the what, the where, and the why. If the hug is coming from someone close to Phoenix, or a child he's acquainted with, he'll gladly hug them back! If it's from someone he's only passingly familiar with, he'll be bewildered, and the character in question will probably receive an awkward shoulder pat and a funny look. If it's from someone he's close to but who doesn't usually show affection (like, say, Edgeworth)... see scenario two.
Kissing this character: Again, it all depends on the situation! A kiss on the cheek from a lady friend would earn a sheepish grin and a red face. A kiss on the cheek from a guy friend would earn an uncertain look and an equally red face. Kisses from strangers... Well, Phoenix isn't the confrontational sort when he's not on a case, so he'd probably just be like "What?? The heck??" Kisses on the lips would be met with confusion, unless he's in a relationship with the person doing the kissing.
Flirting with this character: Lots of blushing. Lots of pretending he's not blushing. Lots of uncertainty over whether he's misreading signs. If it's someone he's romantically interested in, and they both know it, he may flirt back. (Sometimes badly. Often badly, actually. Sometimes intentionally badly.) There is, of course, a chance that he may not realize he's being flirted with.
Fighting with this character: Sure! He's not a physically violent person, but I bet he could be pushed to his limit in the right situation. (Read: threatening his loved ones.)
Injuring this character: Definitely open to it, but anything major (such as injuries resulting in some sort of permanent damage or scarring) would need to be discussed beforehand.
Killing this character: Only if it's discussed beforehand.
Using telepathy/mind reading abilities on this character: With permission, sure! Generally, Phoenix's mind is an open book, but he does have a few secrets. I'd be more than happy to tell you about them!
Abilities: He's good at finding contradictions and pulling information out of people through the sheer power of stubbornness, and he has the striking ability to stumble blindly into things which he knows nothing about and still come out okay. There's also, of course, the Magatama, which I'm open to using in PSLs and games where it's in his possession. Let me know if you're ever interested in doing something with it!
Easiest way to get on his good side: Honestly, if you seem like a nice person, you're probably already there.
Easiest way to get on his bad side: Threatening, hurting, or otherwise being a nuisance to anyone (especially those closest to him), betrayal, partaking in criminal activity with no regret, lacking a conscience... Basically, if you seem like a not-so-nice person, you're probably already there.
Warnings: None that I can think of!