You've mentioned in a few recent asks how Julien, as a character and party member, has been working for you, and you enjoy his character.
Which, same! I enjoy him a lot, too!
Doylist-wise, what would you say are the most important aspects of what Matt's been doing with Julien (narratively, rp-wise, even mechanically) that allows for a character whose got a shit personality and who started the story as an antagonist to most of the main group and Thjazi to remain likeable to the audience as a whole and a good teammate in collaborative rp?
Good question! This is probably going to be a non-exhaustive list, but still, hopefully, pretty detailed. I'm going to split this up into sections.
Backstory: because Julien is an asshole at odds with people he needs to have a pretty involved backstory that causes him to have history with people so that he's not just "that asshole" (even if he is, on some level, that asshole). "Vassal knight to Thjazi's wife AND guy who caught him in the Falconer's Rebellion" is a hell of a backstory with tons to work with in terms of conflict that isn't just "oh he's a dick". He also has connections, however, to the Barrowguard, which means Thaisha and Hal have some shared interests, and in that Julien is someone with a professional relationship with their son Alogar. The fact that Julien is in the Sundered House societal echelon gives him reason to have crossed paths with Occtis and Wick (and, notably, both Occtis and Wick socially outrank him) as well as Bolaire and Tyranny. As someone affiliated with House Royce and fairies, he and Thimble have history; and as a veteran of the Falconer's Rebellion and again, guy who caught Thjazi, that ties him to the Torn Banner (minus possibly Kattigan, who left before the end of the war); we know that Teor and Azune and presumably Thimble were all present at Maharlian falls. Finally, Julien does in fact genuinely care about his family, particularly his sister Alba, and Lady Aranessa; the Barrowguard; honor/duty/etc; and he cares about his parents' approval deep down despite being in many ways extremely disappointing to them. In other words, there's a relationship already present with every character except Kattigan, Vaelus, and Murray. That is more pre-existing relationships than I think most of the cast has, and so it pretty firmly places him in this world and part of these people's lives even though a lot of them don't like him.
Personality pt 1: he is also kind of pathetic. His parents are mad at him all the time and as stated he really doesn't feel good about this; he pretty transparently hates himself, quite possibly for something having to do with his capture of Thjazi; he's canonically considered very attractive but he's mostly going to brothels and he doesn't really have friends outside the Barrowguard; he is hungover for most of the overture and cursed for even more of it because he spat on a dead man. He apparently is a crier when high. You can play a guy who sucks or you can play a guy who is cool but it's really, really hard to play a cool guy who sucks and not play an outright villain everyone genuinely hates, or like, mean girl Mary Sue (or another gendered equivalent).
Note: all of the above is pretty much established from the very start, before we get a subclass, or before the plot really kicks off.
Don't fight the plot: you can take an unexpected direction, to be clear. That's great. But Matt doesn't try to like, stop the giant fucking bomb that drops on Julien in particular (and unlike Alex he did not know something to this effect was coming). He very much reacts in the moment as Julien during the Palazzo Davinos massacre, which gives him a great deal to work with going forward. Obviously, Brennan does a huge amount of the work here by blowing up Julien's whole life (which, tbh, has to happen for all the Sundered House-connected folks for this campaign premise to work out, so props also to Alex, Sam, Whitney, and while we're at it Laura, for having characters who really got their shit rocked by that necessity and handling it beautifully) but Matt yes-ands it at every single turn. It's not the story he imagined - Matt said he'd very much thought the story might turn into reconciliation with Raimond, which clearly isn't really likely (though idk. we can talk to dead people in this campaign) - but he's doing a great deal with the story he does have. I know this is advice often given to DMs but it's true for players; if you want the story to go exactly as you envision it, write a book.
Share goals: If you are playing a guy whom a lot of people find really annoying, have shared goals and be extremely dedicated to them. I've mentioned this a few times but the Seekers level of conflict is like. if that's too hot for you enjoy your unseasoned chicken. It is remarkably tame and a big part of this is that Julien and Thaisha have pretty much the exact same goal re: Alogar/Barrowguard (further cemented once they stop at Castle Torch), and everyone hates House Tachonis. Julien is very confrontational, but he is also very interested in not getting his allies uselessly killed. Which brings me to...
Personality part 2: Julien is the least intelligent of the group but he's not stupid. He is, in fact, brutally pragmatic. This is a great trait to have if you are an asshole, because the thing is, you are also kind of right. Like yeah actually it is important that he and Thaisha know exactly what brings Occtis and Vaelus here outside the meta-level "we are the Seekers table". It's important he know that Dr. Talter can handle being threatened by a man with a sword because she's going to encounter undead. It's important he handle the Einfasens, both Otto initially, and Ingrid's flirtations later, with some degree of diplomacy regardless how he actually feels. I think he walks an excellent line of being mean, but not cruel nor spiteful. He is cold to his mother, but he also thinks he's in the right (and like, narratively, in a D&D game? He might die in a dark hole somewhere, but yeah, marrying Ingrid Einfasen won't save him. He's gotta get a gem from a Tachonis and kill the Tachonis to save Alba or die trying). Matt described Julien after the Seekers arc as realizing this is bigger than just him, and very much plays him that way. Convergence Julien is still a dick but he's not going to withhold information from Thimble, who is undergoing the same sort of grief he is, and instead recognizes her as another person who has lost their home and someone they love, despite their past animosity. He is not holding grudges against the Torn Banner, either, which means he's had excellent scenes with Thimble, Teor, and Azune. He's not going to full Leroy Jenkins with House Tachonis. He even tells Cyd that he is, at this point, effectively a commoner, which is remarkably low on ego.
The meta level of Convergence: some people in Convergence are much more central than others: Hal is very central. Wick and Tyranny are very central. Thimble and Thaisha are both quite important. Murray and Azune have a lot going on in the city. Julien is not one of these people. Matt is playing Julien as though he is not one of these people; like Teor, Kattigan, and Vaelus, he's often just going where they need a martial character as backup.
Finally, Mechanics: Fighter is always a useful class and Julien especially is built to take and deal a huge amount of damage. This means he is pretty much always useful in a fight, which gives people a reason to bring him around (see above). I happen to really like his build and I do think it underscores the fact that he really fucking hates himself, and the whole "booed and living man" vibe which is also a core part of his deal (and is really funny in that he is, simultaneously, built to survive at all costs and also built to uh. ride the edge of death all the time. But anyway point being I do think that if you are a fucking asshole character you better be support or a tank in combat and give back a little, and Julien is indeed a tank.