(no subject)
Backtagging: Yes please!
Fourthwalling: You can go for it, because he won't believe you and think you're insane :V Unless you have factual, physical proof.
Hacking: He's not very technology savvy, so sure!
Threadhopping: No problem! As long as the thread isn't private go nuts.
Physical Contact/Invasion of personal space: Go ahead, but he's not comfortable with it, so depending on who you are he'll react accordingly. But he 100% will likely not respond.
Hugging this character: If he barely knows you he will not appreciate it, but if you're someone he's close to he might soften to it.
Kissing this character: Go for it, but don't expect it to be returned! Also he might lament about his first kiss being stolen.
Flirting with this character/Relationships: HAHA, please do, because he probably wouldn't even notice. Or get really flustered when he did. Also, relationships would take a long time to work up to, and is kind of unlikely as Yukio pretty much thinks he has better things to do than to get a girlfriend. (but who knows, RP makes crazy things happen).
Fighting with this character: Yukio doesn't really get into pointless fights as they tend to be a waste of time (and ammo etc). You can try riling him up, though!
Injuring this character: Ask first please!
Killing this character: Same as above.
Winning/Losing: I'm usually not picky but it might make a difference in context/within plot, so this is something to be talked out as well.
Using telepathy/mind reading abilities on this character: Go ahead, though I'd rather talk about it first! Just to clear where the limits are.