[OOC]
Backtagging: Yes, forever. :|
Threadhopping: I'm gonna say ask first! But I probably won't have any problem with it, myself.
Fourthwalling: Go right ahead; he'll probably twig onto the multiple universes theory pretty early on, anyway.
Offensive subjects: If it's IC for your character, anything done or said IC is probably not gonna offend me OOC.
[IC]
Hugging this character: Go right ahead! He will probably not know what to do with it and just get uncomfortable and try to detach your character, though.
Kissing this character: Go ahead. Same as above re: not knowing what to do with you equaling discomfort. And if it's not on the cheek or something similarly innocuous, he will probably be more irritated. NOT ASKING IS RUDE.
Flirting with this character: Yes. Again, he's... not sure what to do with things like this, and will probably varying degrees of awkward or uncomfortable about it. Will probably also get irritated if you don't drop it.
EXTRA NOTE: Any of the above might, potentially, vary with enough CR. It would probably take a lot, though, especially for the latter two.
Fighting with this character: Go ahead, if your character would want to! The Riddler is less of a physical fighter so he'll try to hang back and find some other way to best his opponent, if that's a graceful option, but if it's not he has no qualms about trying to kill people attacking him. And if he thinks you're dangerous or some sort of threat (even if it's only a threat to his goals), he might try to take you out. (I'll ask before I do anything like that, though).
Injuring this character: ...That said, the Riddler is also not a very good fighter. He's not terrible, but if you have any skill he probably won't last too long. He fights dirty to make up for it, but I expect he'll probably get his fair share of injuries in this game, so hurting him is just fine with me.
Killing this character: It's gonna happen eventually, I'm sure! Just ask beforehand if you really want to kill him/your character really intends on it, and I'm sure we can work something out.
EXTRA NOTE: I find fight logs go more smoothly if I'm in contact with the other player; we don't have to plan out every step, but having a vague idea of how it will go makes me feel more comfortable. If you want a fight, just contact me, and I'll be happy to arrange a scene!
Using telepathy/mind reading abilities on this character: Go right ahead. He will NOT appreciate it if he realizes what you're doing, though; his mind is intensely important to him, and he might flip out at you. He will at least be snippy about it. (Contact me to find out or just verify what you'd find!)
Easiest way to get on his bad side: Insult his intelligence or call it into question. Ruin his "games." Or, no joke, remind him of his father too much. One way to do this is to make sports metaphors, but basically if you are just a certain kind of guy, you will rub him the wrong way so badly that he will project his daddy issues onto you to some degree and will NOT like you.
Easiest way to get on his good side: Respect and compliment his intelligence. Be intelligent; he likes when people can keep up with him (as long as they don't outdo him, that is). Treat him as if he's superior in some way. Play along with him. Or actually understand him (least likely, since he won't be opening up to people again very soon).
Warnings: The Riddler is, not to put too fine a point on it, obnoxious. He's also has a lot of issues he probably wouldn't admit to. You can be perfectly friendly to him and he may still dislike you. And if he doesn't like you, he'll be more willing to stab you in the back (or just leave you in the dust), even if you've done something to help him out. But, in general, just expect smug superiority, stubbornness, bursts of immaturity and fit-throwing, and an ego the size of a large planet.
Other Stuff: Not too much! Physically speaking, he's pretty damn thin and possibly even underweight; he's used to forgetting to eat and not eating the healthiest. If there's anything else, just contact me and ask, and I'll give you the best answer I can and possibly even add to this thing. c: Thanks!