First off, I don't know if this belongs in the Suggestion thread or here, but I want to discuss how I think Larian did not fully exploit the background system in creating open characters that can fit multiple people's interpretations of the companions.
I will tackle Shadowheart first by cross posting this thread from Reddit:
https://www.reddit.com/r/BG3Builds/comments/1avx4z7/unpopular_respecs_that_are_justifiable_from_a/To me this post shows Larian focused too much on describing characters based on their class and not their background. A prime example of this to me is found both on the main page of Larian Studios as well as super late in Act 3 where inside a book itel called "Artefact Mission: Agent Roster" written by Viconia Devir, they wrote of Shadowheart as being just a healer, when it would have been more accurate to refer to her as Magical Support.
This acknowledges the mechanical flexibility of 5e's Cleric class while creating in lore justifications to play Shadowheart as either a Wizard or a Sorcerer because, as explained in the above post, the way DnD 5e's Background system work is that in spite of the environment you grew up, you can whimsically insert a mentor figure to justify having any class you want, despite say choosing a disenfranchised background such as the Urchin or the Outlander.
This could even extend to both Karlach and Laezel but they were written very specifically to be martial combatants, therefore you don't have as much flexibility as with Shadowheart in that, with minor tweaks, you can make her cannon class be ambiguous.
Now on to Wyll. Calling him the Blade of the Frontiers while making it painfully obvious that Wyll couldn't beat a kid without Mizura's aid is extremely limiting. And even bad from a gameplay perspective as you are given at level 1 a weapon with which you aren't proficient and must pass on to Lae'zel.
If Larian would just change his title from the "Blade of the Frontiers" to the "Brave of the Frontiers" it would be much more enjoyable for the players to roleplay Wyll as either a Pact of the Tome or Pact of the Chain Warlock. An even bigger bonus would be if they made a custom background from Wyll who retains his Inspiration Sources. I would go with something like "the Fraud" gaining proficiency in Persuasion and Survival, in order to make picking him as an Origin character less of a hassle to min max for higher difficulties.
To conclude my point I will now provide an example as to how flexible the Background system for DnD 5e actually is, by showcasing two characters I made to help prove my point.
"Nero and Vincent are an odd duo, due to the fact that their luck in life help elevate them above their assigned stations in life.
Nero's mannerisms as an
Outlander are an eyesore to the general public, in part due to his socially awkward behavior or upbringing, but few suspect that he is in fact a very competent
Wizard, especially skilled in the art of Divination.
This however pales with the hypocrisy of Vinnie's beliefs. Once he gets going, it's impossible to stop this scrawny bastard talk about the importance of respecting nature, as he stuffs his face with either sweets or brandy.
At his core he is a street kid through and through, but few know that he is in fact a very capable
Druid, capable of assuming many forms. You wouldn't think it at first because Vinnie is very street smart, something he learned while growing up as one of the many
Urchins of Baldur's Gate.
However, only the locals know that, if not for that Gnome, Vinnie wouldn't have a stable job as an adventurer and would probably be going in and out of jail for petty thefts."
In this example, I tired to make it as obvious as I can that Nero is a Wizard with the Outlander background, while Vincent is a Druid with the Urchin background.
The same methodology can easily apply to Shadowheart, thanks to the fact that a religious cloister can attract a plethora of talented people, in the world of Faerun. This could also extend to the rest of the BG3 cast, except for Halsin, who is a canonized Archdruid.
Larian could sell this concept even better on their webpage for BG3 by altering the titles of the characters. For example they can change Lae'zel's title from Fighter to Soldier and Shadowheart from Cleric to Acolyte and Loyal Warrior to Loyal Confidant.