I've been meaning to do this for a while now. I have Jet's Permission Meme on the DDD Wiki, but I think it's best to put a non-spoiler one here, and a longer, more comprehensive, very much spoiler on on the DDD Wiki. I'll link to it once it's finished.
For now, here's the non-spoiler one. If you have any questions, feel free to comment, and I can expand on any topic you like.
Notes for Psychics: Jet is for all intents and purposes an ordinary human being, but there's something a little off about him. He almost seems too pure, both in a spiritual and physical sense. He possesses some sort of special power, but what that power is, remains to be seen. Psychics will probably be able to sense the spiritual link between Jet and his ARM, especially while it's in use. They will also be able to sense the presence of the Guardians in the back of Jet's mind.
Being an amnesiac, Jet can't remember anything before the age of twelve. But since visiting the Leyline Observatory, he's been having strange flashbacks, possibly of his past. Psychics may try to look into Jet's memories, but many of them will be fuzzy, broken images. The whole thing would probably result in a migraine for poor Jet.
What's Okay To Mention Around Him: There are very few things that aren't, actually. Anything involving money will definitely pique his interest, though.
What's Not Okay To Mention Around Him: As of now, not much (but wait until Jet finds out about his origins, THEN they'll be plenty of angst-fodder). Still, Jet does not appreciate people prying too deeply into his personal affairs or inquiring about his past. He suffers from retrograde amnesia, which means he has no memories of his past. He was also an orphan and apparently abandoned by Virginia's father (Werner Maxwell) who took him in and taught him how to survive in the Wasteland, so he doesn't like talking about anything to do with family. Also, Jet is a bit of a prude and doesn't like getting into raunchy discussions about sex and whatnot.
Feel free to mention any of these things, but Jet's reaction will most likely to be to argue with the person who brought up the touchy subject, or to just stop talking altogether.
Easiest Way To Get On His Good Side: First off, be a mature, intelligent adult. And don't treat him like a little kid. He hates being underestimated. Also, any mention of money or treasure and an offer of a job with good pay will definitely make him warm up to you. Respect his skills and treat him with dignity and he'll return the favor.
Or, you can offer him peaches. They have the same effect on Jet that catnip does on cats. If you have a rampaging Jet charging toward you, throw a peach at him!
Easiest Way To Get On His Bad Side: Be a complete and utter moron. Flaunt your skills, but don't back them up. Get up in his personal space. Treat him like a child. Try and scam him after he performs a job for you. Just be a generally rude and ill-tempered person.
Can I Hack His Journal?: As of now, most certainly! Jet isn't very good with modern technology, so he doesn't know how to lock things properly.
Can He Hack My Journal?: As of now, probably not. He's really not that good with computers, and unlike Clive, he hasn't been trained in their use.
Threadhopping with this character - yes, no, or what?: Unless it specifically says it's locked, feel free to jump in! Just be aware that depending on the conversation and who he's talking to, Jet may either talk to you or tell you to GTFO.
If you purposefully harass him *coughMALIKcough* expect Jet to start CAPSLOCKING AT YOU. He will not put up with your crap.
Fourth Wall Breakage: I'd ... rather you not. At least, nothing specific, especially concerning Jet's origin. I want him to discover all those secrets as we play though canon.
Can I shapeshift/bodyswap/slash/etc?: Only because most of that would be so hilarious, I'd say yes, but just ask me first. (Genderswapped Jet ... too hilarious for words.)
Maim/Murder/Death: For now, I'm going to say no to that, but I haven't ruled out any possibility that Jet might get maimed. If so, it'd have to be non-permanent. Still, he plays a very important role in the world of Filgaia, so it would kinda suck if he died. So, no dying.
Kissing/Hugging/Badtouch: Jet, despite all his worldliness, is actually rather naive when it comes to sex and relationships. He's very distant, both emotionally and physically, so he really likes his personal space. Kissing and hugging is bound to be met first with resistance, then possibly violence. He tends to go stiff and hopes the person will just go away and leave him alone, but failing that, he'll begin to flail and try to escape. Depending on the person, he may very well try to smack them (unless it's a girl, he was taught to be nice to girls).
Badtouching ... your body shall be riddled with no less that thirty brass-plated, fullmetal jacket AP shells.
Favorite Person: He'd never ever admit this, but it's Virginia. HOW BLATANTLY OBVIOUS.
Favorite person he's met on the comm: If he's actually talked to you long enough to form an opinion of you, he probably enjoys talking to you. If he's met you in person, then he probably likes you. He's very reluctant to call people "friends." But, here's a small list of people that probably count as friends: Maraich, Ash Ketchum, Jack Frost, Shinji Ikari, Asuka Sohryu Langley, Kaworu Nagisa, Karen, and he's still on the fence about a few others.
Prefers: Intelligent, mature, low-key sort of people. Overly energetic types run the risk of grating on his nerves (because he'll invariably compare them to Virginia). That's not to say he'd never interact with those kind of people; he's just more likely to butt heads with them. Oddly enough, he has a soft spot for kids, though he tries not to advertise this.