Most people will play as Tav, the custom character, in their first playthrough of Baldur's Gate 3. That's exactly how Larian wants you to experience the story for the first time. However, subsequent playthroughs with either one of the origin characters or Dark Urge will reveal a lot more content for the game.
The makings of a completely different story...
But that doesn't necessarily mean that Tav is the canon protagonist of the story. In fact, throughout most of the game, Tav doesn't really have any character development or arc, and is simply solving their companions' problems. Sure, they defeat the Netherbrain, but that's just because they're the stand in. So, who exactly is the protagonist of Baldur's Gate 3 from a narrative standpoint?
In one of many Reddit posts that theorise who the main character of the game is, Ready_Medicine_2641 suggests it's none other than Shadowheart. "Dedicating your life to a god, only to learn that your allegiance wasn’t your decision at all and your entire life was stolen from you, and then working your way up to not only remedy that but also save the world. Doesn’t get more main character than that."
However, not everyone...