I don't really think Shadowheart is the "main character" of this game. While her arc/story does seem have to some plot relevance via the box, I think that's really the extent of it. She seems more like BG3's version of PF:KM's Tristian, or if you want an example from previous BG games, Imoen in BG2: she's a character who's personal story has some relevance/importance to the main plot but she's not the driving force behind it.
The player character is still very obviously the "main character" and has a ton of agency within the story, even if their backstory doesn't really have anything to do with the game's main plot save the plot hook of being a captured Baldurian.
As for who I'm going to bring with me, if I really had to choose it'd be Shadowheart, Minsc, and a tossup between Karlach and Gale for the final slot. My PC would ether be a paladin, wizard, or rogue. I do hope that they won't lock the party like DOS2 though, it would really be terrible if they did especially since we can permanently lose companions depending on your decisions. They should do what 99% of modern CRPGs do and just have the full party follow you but be at camp/a home base if you're not bringing them with you.
