First things first, I would like to reiterate that I like Astarion as a character and this is not an Astarion hate post.
Second things second, I'm very much viewing this through a DnD lens, as Larian did a fantastic job of making BG3 feel like DnD. So these are observations unique to this game specifically and not an argument about video games as a medium. I know most video companions don't have stories that tie directly to the main plot, but this isn't most video games.
In that same vein, when I talk about the "Main Narrative" of BG3, I am specifically referring to the 3 major arcs of the game:
Dealing with the Grove and the Goblin Leaders
Dealing with Ketheric and the Shadow Curse
Obtaining Gortash and Orin's Netherstones and defeating the Brain
If a quest or moment is tied into one of those arcs, I consider it part of the main narrative, even if it's technically "missable." Them's the brakes in DnD land.
Larian did an incredible job of tying the backstories and personal arcs of the party companions into the main quest, just as a good DM would do for their players. For the most part, each has multiple points of contact to the main narrative, and several are intertwined, which is another hallmark of really good DnD narratives. For posterity sake, let's run down the list:
Shadowheart: connected to the Artifact; connected to the Shadow Curse and Nightsong by virtue of Shar/Dark Justiciars; intertwined with Lae'zel by virtue of the artifact
Gale: connected to the Crown via the Orb and his backstory with Mystra
Wyll: connected to Duke Ravengard, thus connected to the Chosen; intertwined with Karlach via Mizora; mildly connected to killing the Goblin leader by virtue of being the Blade of Frontiers
Lae'zel: connected to the Artifact by virtue of being a zealous Gith, as such is intertwined with Shadowheart; connected to Orpheus either yay or nay, but connected either way
Karlach: connected directly to Gortash; intertwined with Wyll via Mizora
And then there's Astarion. Who isn't connected to jack shit. With the exception of the encounter with Araj Oblodra in Moonrise (sort of), Astarion's only narrative threads are Cazador (and even the Araj encounter is pretty directly descended from his Cazador trauma, though it's more focused on his self-actualization, so I give it half).
Cazador is wholly removed from the main narrative of the game, and even just the main stuff you're doing in Act 3. Everyone else's main Act 3 quest is at least tangentially connected to your main objectives, with Shadowheart returning to the Cloister probably being the next furthest.
Karlach is killing Gortash.
Gale is killing the BBEG to get his hands on the Crown.
Lae'zel is getting into the Prism to free/kill Orpheus.
Wyll is rescuing Ravengard.
And then you've got Astarion dragging you completely out of the way for an encounter whose only relevance to the main plot is whether or not the Gur are gonna ally with you in the final battle.
And that's incredibly weird to me. Larian very clearly put a lot of intention into connecting these characters to the greater narrative, so the fact that Astarion isn't feels like a sore thumb.
And what's weirder in my opinion is even the non-origin party companions have stronger ties to the main quest than Astarion does.
Halsin is intimately tied to both the Goblin Leaders and the Shadow Curse (though admittedly he's actually usable for very little of it).
MInthara is connected to killing Ketheric sort of but to killing Orin very much so.
And Jaheira and Minsc (who are so intertwined they're basically one character in Act 3) are connected to Orin twice over, once by virtue of the Zhentarim/Dopplegangers stuff and once by virtue of Sarevok. (Jaheria is also obviously connected to the Shadow Curse and Ketheric in Act 2.)
That's what makes this so weird to me. It isn't "oh Astarion is the only origin character without ties to the main plot," it's "Astarion is the only party companion period with no ties to the main plot."
Astarion is the player who had a really compelling backstory they wanted to play but just had absolutely no way to work it into the big stuff, so while everyone else is focusing on progressing the narrative, Astarion is busy brooding and doing mischief and talking about Cazador (who, again, is a completely unrelated and off-screen threat until the literal moment you're killing him).
I don't know why. Sometimes it feels like Astarion's only purpose is to be there for people who want their Tav to be more morally ambiguous or outright evil so you have a companion who doesn't judge you. Hell, sometimes Astarion feels like Tav with a backstory. Like he's the main player character in any other game but accidentally became a companion in this one.
I know that Cazador's palace was originally supposed to be much bigger and have much more going on, but that doesn't really feel like it changes anything to me. Unless Cazador was originally connected to the Absolute somehow, simply having more Cazador content on location does nothing to connect Astarion to the main narrative.
The only way you can really even try to weasel Astarion into the main narrative is by connecting him to Raphael, but his deal with Raphael to learn about the scars is so one-off and cut and dry that it really has no long-lasting impacts, and even if you don't kill Yurgir all that does is push back the timeline on when you learn about the Rite of Profane Ascension. It doesn't actually change Astarion's arc even a little.
Now at the end of the day, I don't really think any of this means anything. It doesn't make Astarion a bad character and it doesn't in any way invalidate his personal narrative. And if this was any other video game, I really wouldn't give a shit. But because it's a DnD game and because Larian did such a good job of modeling it after real DnD campaigns and because literally every other party companion has ties to the main narrative in one way or another, it just really really sticks out to me how narratively meaningless Astarion is (in relation to the main quest only).
If anything, I think it proves just how good Neil Newbon's performance was, that Astarion is such a beloved character despite being almost completely irrelevant in the scope of the game's story.
I think it also really rings true the Oscar Wilde quote: "It is absurd to divide people into good or bad. People are either charming or tedious."
Astarion is charming. Wyll is tedious. (Or Lae'zel, if you ask me.)