Suppose I might as well add a few thoughts as well.Am I late for the party? Eh, here's my two cents. Just so you know, I'm gonna be defending Tommy here can't believe I've found myself agreeing with Dav
1. Here I was supposed to be on Tommy's side, but this one is where I mostly agree with you, Geralt. Shooting up hard drugs never leads to a good outcome, m'kay? Although I still wanna point out that it is ultimately Tommy's choice/right to put whatever he wants into his body.
2. Perhaps, but the thing is, he's MC's frat brother. And who is Heather to Fuckface? A nobody. So why would anyone hold against Tommy something, that is a part of his private/personal life? Ain't my business what's going on in that relationship.
3. Ah, here's the juicy part. What is this "secret code" at its core? A seemingly convinient and more than a little arbitrary method to sweep the dirt under a rug that a bunch of dumb college kids came up with in order to protect themselves whenever they fuck somebody they shouldn't have fucked in the first place (or when they do some other dumb shit). Like with Josy, MC knew all too well how Tommy's gonna react to that revelation, and he still didn't end things with Josy (and Maya). He was careless and, if you ask me, inconsiderate of Tommy's feelings. And I'm not even gonna cover the Derek bit, that shit was for laughs and shouldn't even be considered a "violation" of the code.
How did Lily put this? Something like, "aren't codes more like a bunch of guidelines that everyone cherry-picks from?" Hear, hear.
4. He could've handled it better, true. Maybe he could talk to Trem after he calmed down and clearly let him know that Vinny is one crazy son of a bitch. But right after Trem's altercation with Vinny, our Snowflake McFuckface was shaken up so dad he was ready to call the cops. I believe I don't need to remind you of the consequences of that for the whole frat, not just for Tommy.
5. Personally, I'm in the "Tommy cares about Josy more than he lets on" camp, so I would attribute that to his overprotectiveness. I might eat my words later, who knows? But for now, I'll keep being optimistic.
6. Who isn't and wouldn't be jealous of that guy? In a couple of months, he became the most popular guy on campus. That kind of success breeds jealousy even in the most unenvious people. And frankly, MC needs someone to be skeptical of him. We don't want Trem's success to get to his head, do we?
I guess I can relate to him somewhat. He's the guy whose temper and bad habits tend to get the better of him, despite him not being a bad person, really. Let's look at his record so far: did a shitty job as a father? Came around, apologized, and made an effort to make things up with MC. Threw a fit after the mansion was trashed? He calmed down, started doing his part, and found a gig for the DIKs to make some more money. Yelled at Quinn for ruining the evaluation party? He admitted his part of the blame, apologized, and gave Quinn a chance to do the same. And so on, and so forth.
This is a personal thing, but I tend to prefer this particular character trope Tommy embodies so well: Jerk with a heart of gold don't shoot me for being cheesy, that's just how they named the trope on TVTropes!. Cripples, bastards and broken things tend to be the most amusing people for me, both in fiction and IRL.
2. Regardless of who Heather is to the MC, how Tommy treats anyone that he's in a relationship with is going to influence MC's opinion of his character. To take it to an extreme, imagine if Tommy was routinely physically abusive to Heather. Do you really think that shouldn't influence how MC feels about Tommy's character quality just because he's a "brother" and Heather is someone MC only somewhat knows? A pattern of cheating is a form of emotional abuse; if I was MC, you can bet your ass I'm thinking less of Tommy for doing it, even if I don't get involved or intervene in the relationship since it's a private matter.
3. Regarding Josy and MC's relationship, it would certainly have been more considerate if MC mentioned it to Tommy much earlier on. That's on him, though he did have some extenuating circumstances since 1) he met Josy months before he met Tommy or knew they were related and 2) Maya is involved as well and shit could go bad for her if too much of their relationship goes public. I find it interesting though how for Tommy/Heather's relationship you say it "ain't my business what's going on in that relationship", but for MC/Josy, you imply that they should break up just because it might make Tommy uncomfortable. It feels like there's a bit of a double-standard there just because they're step-siblings, and that you're taking away from Josy's personal agency (and I say this as someone that has a younger sister in real life, with whom I have a much better relationship that Tommy and Josy).
5. It's certainly possible that he's overprotective of Josy. But it's easy for overprotective actions to slip into controlling actions instead. Feeling that MC shouldn't date Josy just because he's a DIK without considering how Josy feels and/or whether or not the relationship is healthy and good for her isn't being protective, it's being controlling. With that said, a conversation could be had to consider how if the relationship goes south, how is that going to impact MC and Tommy living in the same house etc.
Tommy does have some redeeming qualities and occasionally does show that he cares about people. Having mixed qualities like that makes for an interesting character. But with that said, just because you might have a "heart of gold" (and from what I've seen, if he has one, it's pretty underused so far) it doesn't mean you can't also be an asshole.