Okay. First off:
Edward Elric: Ah, they really don't interact much except for a few very brief exchanges. I kind of get the impression that Havoc sees Ed as a bit of a pest and I'm of the opinion that Ed's 'Boss' nickname comes from him being a total bossy brat to the colonel. |D He does respect Ed's abilities though, same as he would any other state alchemist. And he does like him in an annoying nephew kind of way. Since Ed and Winry have repaired the damage to his spine, Havoc feels deeply indebted to the kid. He knows Ed's not the sort to accept any kind of repayment, but Havoc will probably go out of his way to help him when he can.
Alphonse Elric: Less canon interaction than Ed. I can only recall one instance where they were doing something together, and that was just a single panel with Havoc helping Al after his armor was damaged. I figure Havoc likes Al same as anyone who gets to know the kid does, but they won't be interacting much cuz talking to myself is boring. |D
Envy: Havoc never met Envy in canon. Mustang's spoken with him about Envy since he arrived in camp though, so Havoc knows he's definitely someone to look out for. He's aware Envy is Hughes' killer as well. Even so, they've yet to actually meet in camp either. |D Go figure.
Fletcher Tringham: Generally a nice kid. Has good intentions and tries to be friendly and all that good stuff. Havoc thinks he's a bit annoying with all his talk of different versions of Amestris and who's from which one. He just doesn't care about that kind of thing.
Russel Tringham: Havoc's a bit confused by him. He doesn't get why the kid talked to him about things he didn't want the military knowing about when, in his mind, it was pretty obvious that he was still loyal to the colonel. He likes him though. Thinks he's got the right idea worrying about Meer over Dullindal.
Winry Rockbell: Never met in canon. Adores her in camp. She's one of Havoc's kids - his counselling group. Havoc thinks she's a sweet girl with a good grasp of who she is and what she wants to be. He likes talking with her. Also, she and Ed worked together to repair his spine - there's really no way to thank her for that. Especially since she wouldn't let him.
Alex Louis Armstrong: As long as Armstrong doesn't try to hug him, they get along fine. Thinks he's kinda crazy the same way everyone else does, but generally likes him.
Izumi Curtis: Never met in canon. Doesn't mind her dislike of the military, though he hasn't talked with her at length about it yet.
Maes Hughes: Ackpbbt he's alive whut. Thrilled to have him back same as everyone else. Just as irritated by his family obsession as everyone else, but will totally put up with it because he's Hughes.
Riza Hawkeye: Partner! ...they really don't have a lot of interaction in canon though. Havoc doesn't interact with many people. D: BUT STILL. Seeing as they're both in Mustang's most trusted, it's safe to say they get along I imagine. Like everyone else, there's lots of respect for Hawkeye's ability. At a point during a mission where Havoc's about the get pwned by the enemy, he has no doubt that Hawkeye will nail the guy from her sniper spot - which she does. So yeah. Lots of respect and friendship there.
Roy Mustang: Colonel~ I don't think there's any way to properly convey the levels of trust and respect and all the other thing that exist here. Mustang says he's going to be fuhrer, and Havoc believes that. He'll do whatever is needed to help him get to that position. He's willing to put his own life in Mustang's hands, because he knows the colonel will do everything in his power to keep it safe - the incident that paralyzed Havoc's legs proved that of the colonel. And at the same time he knows Mustang can be a complete idiot at times - wtf you can't use your gloves in the rain colonel. D: And he's completely willing to pick on him for it, and for anything else he does that Havoc happens to think is stupid.
And now, in no particular order,
Tsunade: Real nice doctorin' type. They were introduced by Mustang who wanted her to look at Havoc's spine. They haven't really spoken since that first visit, but Havoc still thinks she's a good woman who knows how to do her job, which is always worthy of some respect.
Heather Morris: Flirted with her a bit until he found out she had a boyfriend. Appreciates that she didn't rip into him for it though. Good kid.
Urameshi Atsuko: Doesn't know why she doesn't like her kid. Also doesn't know anything else about her.
Ari: Good kid. A bit too hard on himself, but he's trying and that's gotta count for something. He's also another kid in Havoc's counselling group.
Uzumaki Naruto: Met once. Seems like a decent kid.
Xiao Qiao: Real cute. She gave Havoc some warnings when he first arrived, but they haven't talked since then.
Anne Shirley: She's a sweet girl. Havoc promised her that he'd work on finding a way out of camp, and she seemed pleased with the reassurance. He'll try to look out for her more in the future.
Kudou Kazahaya: Met him while he was Rose Brided. WTF.
Otomiya Haine: Also met while Rose Brided. WTF again.
Anakin Skywalker: Kinda startled to see someone so old as a camper. Thinks he's a reasonable guy with a good head on his shoulders. Wouldn't mind getting to know him better in the future.
Malcolm Reynolds: Havoc apparently reminded him of Wash at first. Overall, Havoc thinks Mal's a good guy. He gets along well with the colonel and Hawkeye and looks after campers the way counsellors should. He does find him a bit annoying every now and then, but it's all in good snarky fun.
Ash Ketchum: Dot dot dot, runt. For whatever reason, Havoc likes the kid. He's energetic, friendly, makes attempts at being helpful, and is a complete idiot. He goes out of his way to look out for Ash and carries on long, rambling conversations with him. Usually they talk about nothing, but every now and then they have a serious talk where Havoc gets to give real advice. Consider Havoc your adoptive parent, Ash.
Tom Riddle: Vaguely aware of in that he knows there was some sort of interaction between him and Ed. Doesn't know very much about him though. ETA: Turned Yzak into a rabbit - wtf. Not angry about it, but is kinda curious about him now.
Kagurazak Asuna: Cheerful girl. Was a bit concerned about her brush with some tentacle pokemon, but isn't too worried now that she's gotten rid of it.
Uchiha Itachi: Met him while he was a girl and proceeded to hit on him. Apologized when he turned back into a guy. Havoc thinks he's a pretty good guy seeing as he didn't flip out on him for flirting with him.
Hyuuga Neji: Aware that he's got some connection to Hinata. Thinks he's a good kid.
Hyuuga Hinata: Another of Havoc's kids. Havoc thinks she's adoreable, though he hasn't seen her since she regained her normal age.
Aburame Shino: Likes the way he looks out for his teammate, Hinata. Thinks he's too quiet, but definitely a good kid.
Inui Sadaharu: Strange kid. Obsession with data can't be healthy. Doesn't really mind him though.
Gilbert Dullindal: Heard from Russel that he'd had Meer killed before camp. Definitely gonna be wary of him.
Meer Campbell: Only knows through Russel - apparently killed before camp.
Yozak Gurrier: Married to a bunch of the A Team and others - the hell? Gonna have to actually meet him some time.
Nicholas D. Wolfwood: Havoc likes Wolfwood. He seems like a pretty good guy who knows where his own shortcomings are. So he killed a camper before - he does show regret for it and doesn't want it to happen again. Havoc'll keep an eye on him, but he's really not worried at all.
Yzak Jule: The last of Havoc's counselling bunch. He's got a temper, but it really just amuses Havoc. He likes teasing him about it just to get a rise out of him. Aside from that he seems like an okay kid.