The issue is Wyll was rewritten too close to release. It's pretty much the same with Halsin, who was only included last minute and as a result is very bare bones in dialogue and quests. A lot of care and effort went to the female characters, with multiple Acts spanning their story arc. The only male character who got similarish treatment was Gale. Wyll and Astarion got very slim pickings, though Astarion at least gets some interaction with Raphael in Act 2. But Lazael and Shadowheart just eclipse the other companions by a mile, and Karlach has enough of a loud following that she's getting fixes just like Minthara and probably Astarion will.
Honestly, Lazael should have gotten her time with Act 1 at the Creche, Shadowheart her Act 2, and the other companions should have gotten more emphasis in Act 3 to compensate. But the story just stayed mostly Lazael and Shadowheart focused with Karlach in third place.
The rewrite would explain how barebones his story is to the others. To+ be honest I think Astarion gets more than enough content in the game to rival Shadowheart, especially considering he's not really all that important to the plot in the grand scheme of things. He also just gets a lot of situational dialouge that the other companions just don't get. Like if you bring him to the creche and sun tan him, he'll make a comment on it, something that none of the others get to comment on.
My feelings on the rewrite are Larian says they did it because they apparently wanted to do right by the character, but it seemed as though not a lot of care was put into it. If there wasn't enough time to do it properly maybe they should've just stuck with what they had at first back in the EA. It also still doesn't explain his lack of relationships in the city either. Did he ever have any of those planned? And then there's the matter of the art book and his lack of concepts in the book.
Halsin was a character they squeezed into the story for fanfare. To be honest with how he's written, I do think he should've been just a fling you could have in Act 2. Similar to the drow twins.
