(Not sure I’m the best for advice, and I think kuroba-k gave a good answer to your question, but I guess I don’t mind offering my own input.~ Not as nice and neat and to the point, though…)

(First thing for any RP blog would be know the rules and basic manners of rp, such as no godmodding, coming into something like a relationship is already in place, etc. Might not be relevant to you specifically, but have to get that out of the way to start just in case.

Second: Know your character. Those might just be my thoughts on him, and you don’t have to go into near as much detail, but at least have read the manga. You don’t have to know every single little detail, but knowing how he’d react to most situations, even ones he might not be used to, help. Every interpretation of him is slightly different as well, depending on how you view why he does certain things, so to get the best idea for your Kaito… well, just ask questions, really. And answer them. Make headcanons. What do you consider the title of Kid to be to him? He was clearly attached to his father, but how much did Toichi being gone really affect him? Do you think his normal personality as Kaito is real, or another act? Is what you want him to do more along the lines of the somewhat more believable recent MK, or back to the slightly more insane comedy manga chapters, where anything can happen and magic might run rampant? Is he lonely, how much attention does he need? Has he matured at all, or is he still the jerk who peeks in the girls locker rooms? Does he really follow guides to make sure no one gets hurt, or does he just not get violent in general? There are numerous possibilities that could change from Kaito to Kaito, which is why there are so many. He’s a complex character, not difficult to play, but has so many different ways of looking at him that there isn’t actually any set way.

I do think kuroba-k made good points on him as a character and how to treat him in general, so I’ll leave theirs for that, but I do want to say one thing. Kaito is fairly social, but can also retreat, fall back and be quiet when he needs to. He’s a character that’s fairly open to interaction in comparison to others, so really, things like knowing where he’d be, what he might be doing, is he shopping or staking out for a heist or coming home from school really go far in rping him.

Of course, I just knew the manga and had the character in my head when I made this blog, without much else going for me, so I can also say that, even if you don’t have the clearest idea, if you know the character, they’ll grow. You get the most solid grasp when they start interacting with others, so just… I lost the point I was trying to make there, but really, don’t feel like it’s hard, I guess. ^^; )