Out of Character~Backtagging: Forever...or until loss of interest which ever comes first.
Threadjacking: As long as it's not a private thread, then feel free to butt in!
Fourthwalling: Iiiiiit depends. You can lean all you want but don't go busting the wall down with a sledge hammer. It's probably best to ask first.
Offensive Subjects: Nope, nothing I can think of.
In-Character~Hugging: Um...you can try? But why would you?
Kissing: Confused looks. Confused and shocked looks everywhere.
Flirting, Romance, Sex: Flirting is fine. As for the other two, are you Chane? If not then please do not pass flirting. Though it is okay if you like...have a crush on him. He's not going to return your feelings, but it's still fine.
Fighting: HA HAHA HA HA HAHAHAHA! You. Fighting. Him. You sad, sad person who will either be beaten to a pulp or not fought at all because he just doesn't want to fight you. (Yes fight him to your heart's content, you poor bastard. No permission needed.)
Injuring: Why not. It's going to be hard, but it's not entirely impossible. His ear was nicked by Chane when he wasn't paying attention. He was also grazed by Spike's bullet when he shot at Chane because once again he wasn't paying attention. Another example is in 1933 he was shot in the arm by Christopher thanks to it ricocheting off some support beams. Granted he wasn't fighting at full strength and probably took the hit on purpose to prolong the fight just so Chane would join in, but it still doesn't change the fact that it happened.
Also as a side note it's not like the guy can feel pain. He has either trained his body to not feel it or he's like Shizuo and just doesn't feel anything. So if you're trying to hurt him to inflict pain then...yeah...not gonna work. Not that he would let you do that even if it did...but still.
Killing: HAHAHAHAHAHHAHAHAHAHAHAHAH!!!! (I'm evil, so yes you may try. But it will be even harder then simply hurting him.)
Mind Reading and the like: Meh, read his mind with in reason. He's pretty confident and optimistic on the surface, but he has deep, deep, seriously deep issues. For example:
[Regarding Chane] "You and I have similar eyes. You don't know where you should release your emotions, so you pile it all up inside yourself."
He laughed abashedly.
"The only thing in this world I can't do anything about is this idiot called 'me'."
This was why Claire had turned all of the bloodlust borne of tragedies and hypocrisies towards himself. He had sealed the bloodlust emanating from his eyes within himself throughout the course of his life.
- 1931 Grand Punk Railroad (Express Episode)
He's also lost like three parents while he was still fairly young. His mind is either a jumbled mess, or you're going to have to dig through all the "world is mine" stuff for the real juicy issues.
It's also okay to get a hold of him through telekinesis. He comes from a universe where certain types of telekinesis are canon thanks to homunculi and certain telekinesis having mythical creatures.
Other Information~( IMPORTANT!!! PLEASE READ THIS! )