OUT OF CHARACTER
Backtagging: Sure. I'm willing to back-tag for as long as the other person is willing.
Threadjacking: Sure, if it's okay with the other person I'm tagging as well.
Fourthwalling: Go ahead, he might fourth wall as well.
ACTION
Fighting Sure, but it wont be much of a fight without Soubi. Ritsuka's a pacifist and would just rather not fight.
Injuries Go for it. He's used to taking a hit. Just let me know what you plan to do first.
Killing I'd rather not, but if it fits, then maybe. Talk to me first regardless.
Telepathy/mind-reading Go ahead, just let me know before you do.
RELATIONS
Hugging You can. Depending on who you are, he might just shove you off, blush or yell at you.
Flirting Go ahead. Most of it will probably go over his head anyway.
Kissing and/or other intimate actions You can, but again his reaction depends on who you are. He'll either get angry and call you a pervert or blush and call you an idiot. Anything more is a no. He's only twelve after all.
Anything else you'd like to make known? He has cat ears and a tail. In the world of Loveless they're a sign of ones virginity, so feel free to gawk at them.