I can summarize most of this right now: If it happens naturally from the characters, I'm down with it. When in doubt, talk to me!
With that said, onwards to the tl;dr!
① Heart?
"Axel told me that if we had hearts, we'd be able to understand all sorts of things. I really don't understand what a 'heart' is."
At this point in time, it is debatable whether or not Roxas has a heart. I am going to try and keep it vague whether or not he does have one in play, just in case I need to make a mad dash towards canon one way or another. But despite being a Nobody and therefore techincally not supposing to have a metaphysical heart, he certainly acts emotional...
② Triggers
but i'm thinking of what sarah said
The only thing you've got to trigger is his anger and his emo, really. It's not hard to tap into either of them. XD If you want to piss him off, be a jerk. Especially to his friends. Especially to Xion. You make Xion cry and he is hunting you down. If you want to make him sadface, just make him feel bad about being him. Put him down when he asks questions about mundane things ("What's my job during vacation?"), treat him like he's a weirdo because he doesn't have a family or remember his past... Generally? Make him feel unwanted and like an outcast, and he'll back off to go woe in a corner to his diary. Uhhh. If you want to make him happy, give him ice cream? XD
③ Medical
[Herein we will determine your fitness for future missions by conducting an assessment of your combat ability. Prove your endurance by...]
He's a healthy kid. *shrug* He's in great shape thanks to his training and missions and he has some decent muscle on him that lets him add firepower to whatever he's swinging, but otherwise there isn't anything out of the ordinary. Although he is a Nobody, he does have a physical heart.
④ Mental
"Axel says girls are complicated. You can't press the wrong buttons. Like I'm supposed to know what the right ones are."
Well, uh. Have I mentioned the part where he seems to have amnesia? Yeah. He has no memories prior to his joining Organization XIII, and even the early days are hazy for him. Because of his special memory situation, he doesn't know about a lot of things. Like, you know. He didn't know what love was. But he knew that Lumiere is a candelabra. Hmmm. Well, I've decided that when it comes to objects, he generally knows what they are, though he may not know specific words for things (does he know that books have spines? hmm). However, concepts are a completely different matter, especially when it comes to emotions. He's still learning about the world, and it's in his nature to question everything. So, yes, he's going to be that annoying kid poking around going "What's this? Why's it called that? I don't get it. But why?" all the time.
Otherwise, he's relatively healthy, mentally. He hasn't reached the point in his canon where he has anger issues, and there hasn't been much to be especially upset about. Just a few weird dreams and maybe some girl problems now and then. He wants to be with his friends forever! So he's pretty innocent about a lot of things. Okay, well, most things. Unless it involves fighting. He's gotten pretty good at that.
⑤ Fourth-Walling
don't let me second guess what i know to be real
Eh, why not. He won't understand or believe it, anyway. XD It's in Roxas's nature to believe in his own existence. He thinks, therefore he is. To him, it's not any more complicated than that.
⑥ Touching
[Saïx approaches. Xion stares at the floor and Roxas holds her hand.]
Go for it. It'd be nice to have a heads-up, but it isn't necessary. Roxas is 100% derp and completely unused to friendly physical contact, so don't expect him to start reciprocating automatically. It would be nice to avoid the badtouching, though. I'm afraid I'm not playing the game for shota molestation. :)
⑦ Romance
"If I had a heart, you think I could love somebody?"
I'm not sure if Roxas really understands what romance is. He'll probably have trouble discerning what makes a girl/boyfriend any different than a best friend besides the physical contact barrier (and he'd only get that part after it's explained to him). If something comes up between your character and Roxas, we'll deal with it then. I'm kind of a big fan of letting things unfold naturally from the characters, so if it happens, it happens, and if it doesn't, oh, well, there's always crack fic. XD I'm not saying "yes" or "no" to this; I'm just saying that there'll be a lot to work on (and through) should we reach that point. Especially since he's a Nobody...
⑧ Sexuality
i am what i am
Not that it's particularly important, but, uh. I'm going to say that Roxas is, on default, heterosexual. I'm not sure if he even knows what homosexuality is, or whether or not it's okay to like boys instead of girls, or both boys and girls, or anything like that. But he's going to just kind of automatically assume that boys like girls and girls like boys. No particular reason why. And that doesn't mean that he is concretely heterosexual; I don't really know. He hasn't had a reason or chance to really explore anything, so we'll just have to see what happens.
⑨ Fighting
"I have to know who I am... I am DONE WITH THIS."
Hahahahahaha bring it. <3 I can't promise I'm the best at RPing fight scenes, but I'll do my best. Roxas's Keyblade is, on default, the Kingdom Key, unless he knows the fight is coming and he prepares his panels for it. I'll definitely let you know when he does because I'm a huge nerd about things like that, though. Anyway! If your character wants to pick a fight with Roxas, they can go ahead and try it. XD
⑩ Psychics
dismantle me down (repair)
Uhhh, well. In general, there's nothing all that interesting on the surface, really. He's just your average teenage kid, even if he lacks conscious memories of his past. However, if your psychic character is to dig deeper, they'll probably notice the broken chains of Sora's memory scattered here and there, but they're not very strong and they only rise up occasionally when something triggers them. Again, that's just in general. For more specifics more suiting the situation at hand, get in contact with me and we'll work stuff out.
⑪ Hacking
"The three of us ate ice-cream together. The sunset was beautiful. I'll never forget today."
lololol go for it. Roxas isn't the brightest block on the gummi ship, so it's not like his password is ~super unique~ or anything. (For everyone's future reference -- including mine -- it's seasalticecream. :| Like I said, he's not so original.) He is also technologically challenged like all Soraspawn, so if he tries to lock anything, there's a good chance that it's hackable.
⑫ Threadjacking
for once i want to be the car crash, not always just the traffic jam
Preferably, drop me a line first and I'll clear it with whoever else I'm threading with. But if it's just an open post and nothing serious is going on, go ahead and pop in. XD A heads up is always nice. However, if the thread at hand is serious, I'll have to ask you not to threadhop freely. If you have a plot idea about jumping in, by all means, go ahead and ask, but otherwise, I'd like to keep the srs stuff srs. <3
⑬ Backlogging
time here all but means nothing
Yesplease. Attention spans and other RPs and difficult muses and work and school and family and friends and yadda yadda yadda, life gets in the way. I kind of love it, actually. I can roll with a thread forever until it closes, and obviously that requires some allowance for backlogging. XD
⑭ Backdating
"Wibbly-wobbly, timey-wimey..."
Noplease. Preferably not. If it's something un-srs, I'm sure it's fine, but in my experience backdating tends to make my head hurt. Dx Because then we have to make the ending match whatever is happening currently and it's just. Uwaaaa. If we absolutely must backdate something, we'll manage somehow and I'll just have to not be a silly nervous person about it. XD But taaaalk tooo meeeeeeeee.
⑮ Miscellaneous
'cause i am falling into grace to the unknown to where you are
Any questions? Anything I didn't cover? Respond to this and I'll get back to you asap. <3