How to deal with player who won’t roleplay, insists character-friction is bad, and doesn’t take the game seriously?

Edit- I’d browsed most of problem-players and still felt we needed more info, or more allegorical advice from experienced gamers but am now seeing stuff in gm-techniques I had missed that is somewhat relevant, if this question is redundant then I won’t begrudge it being moderated and locked or deleted, but I would still like to request some more specific advice.

I know this is a bit of an amalgamation of other similar questions on here, but this feels a little more of a specific case. The GM encouraged me to post this here to try and get some advice since we’re both kind of stumped.

I’m a player in a 3-person group (four including the gM) playing the modern Conan game. Three of us are close friends and the third party member is the girlfriend of the other party member, which is to say that this is a tight group and "booting someone from the game" is basically the nuclear option where me and the GM would potentially fracture off and form a new group with people we know, the least preferable option all things considered.

We’re continuously getting in to arguments about how the game should be played and how roleplaying and mechanics should work (specifically the more vague "non-dice roll" mechanics). The problem party member is consistently at arms with me when I insist that we should be in character more often, or that I’m not "acting in character", or that he doesn’t understand my character (which the GM understands very well, it’s just not a one-dimensional character like his is winding up to be), or that "it won’t be fun if your (my) character has a fleshed out backstory and the GM just turns it into an adventure about your guy."

The problem here is that the GM has pretty specifically stated that this is a "Roleplaying" game and the entire purpose of us playing this is to roleplay. When I (and the gm backs me up) suggest we should try and stay more in character, and use out of character chat more specifically, he basically said that it’s "cringe and I’m not going to go "OOC!!!!!" to talk about things." The problem here is that him and his girlfriend spent literally an hour and a half arguing over a plan in a very split second sort of hostage negotiation where I was set aside to ambush in case things go wrong. After everyone was clearly getting fed up with him trying to hatch some scheme where we come out on top with every reward and absolutely no bad things on our end I roleplay my guy coming up and basically ending the negotiation preemptively with a demand and ultimatum. I do this without consulting them, hoping to throw in some "surprise" to the entire thing. This goes over horribly and results in a huge fight over "we’re a team and we HAVE to always work together." The girlfriend has never played a tabletop and is sort of only here because he is, so she takes his side heavily and insists we have to always have perfect group cohesion or come to a mutual agreement before acting.

The problem with this is that the problem player in question constantly acts of his own accord without consideration for anyone else, but he’s almost never actually roleplaying. His character so far has consisted almost solely of acting like a toddler (playing "keep away" with an amulet an NPC is demanding back that he had stolen and acting confused that she isn’t "friends" with his character despite the fact that we basically murdered the entire group she was with before this). Of note is that he basically kidnapped this NPC for no in character reason just to "see what would happen."

During this amulet keep away, I once again try to have my character swoop in and grab the amulet away and toss it to the NPC and give him some light knock on the head and a quip about "If it’s baubles you want, I’ll treat you to some, let’s keep moving." I tried using advice I’d found in similar questions on here to encourage him to roleplay the situation, or do a pvp skill test to resolve the situation, but he got livid and insisted it was terrible and both him and his girlfriend compared it to "stealing from player characters at night with no consequences," a thing that both the GM and I rebuked were not remotely similar at all, as one was directly encouraging roleplay and in-group friction that isn’t necessarily conflicting. I even offered a friendly "out" for him to benefit by offering to buy him things at the market.

Because of him acting like this, and constantly challenging both me and the GM (for instance his character sheet was very wrong because he didn’t read anything in creation and just sped through in 15 minutes [it took me 4 hours and even took the GM like an hour to make a random gen character], but he got pouty about the fact that the GM said he had to rebalance it to be not broken), his girlfriend is basically staying silent and not roleplaying at all, despite having implied that she was interested in trying to get in to her backstory.

In addition, the GM has specifically requested us to give him a fleshed out backstory for incorporating story elements along the lines, to which he insists is "not fun and it’s more fun to make things up and develop our characters from a blank sheet as we go." Which is to say he basically wants to play a video game where nothing matters.

The biggest problem here is that he is the one who was the most insistent on wanting to play a tabletop RPG, but he both says he doesn’t want it to be "combat focused", but also consistently refuses to engage with roleplay in any meaningful way, and actively fights against it if anything happens that isn’t the literal perfect outcome for his character outside of dice rolls.

Sorry for the long post, but this feels like a pretty complex situation considering his girlfriend is constantly getting annoyed with him despite her always taking his side on things that the GM takes my side on, and it’s ruining the fun for everyone in different ways. The GM constantly has no idea how to handle the situation because his character is acting like a toddler, his girlfriend is bored out of her mind because he constantly overtakes any attempts of ours to have character agency, and I’m having no fun because I’m constantly being told to just not play my character and be a mindless video game companion character.

Is there any way for us to salvage this situation? Any last ditch efforts to try? Do we ultimately just have to have the GM message him and say "this is how I want to play the game, please take it more seriously or we will just have to stop"? The GM set up scenarios that will hopefully encourage him to roleplay, and I’ve been trying to stay in character (and the GM says he will force an in character resolution, instead of bending to his will that it’s not what he wants to happen, or at least a dice roll one, the next time there is friction between his and my character) in order to try and encourage him to roleplay.

Edit for clarity and as a TL;DR – The problem here is basically that the player refuses to actually get in to character, insists that "roleplaying is cringe" despite being the one who specifically wanted to play this game and said he didn’t want it to be a "combat exclusive" adventure (ie lots of roleplaying). He’s hostile towards anything not going his way or any "trolley car" situations he can’t BS his way into derailing the trolley car to save everyone and also find 5 gold on the ground. Any friction (that the GM says is good and interesting) between player characters is taken as a personal offense by him and leads to fighting between me and the GM and him and his girlfriend. He refuses to develop any sort of backstory for his character and is dragging down the entire party, but the group will stop existing without him since his gf would leave, so we’d like suggestions on how to push him in to actually roleplaying / taking things a bit more seriously and treating it less like he’s the protagonist of Oblivion or something.

The GM is linked to this question per his request, so any GM specific advice for him or player specific advice for me are both welcomed.