What follows is a breakdown of the stories of each companion boiled down to a sentence in no particular order.
Because pretty much everyone here can be described with 100% accuracy in one sentence.

Companions.

Gale : Is a wizard that did something stupid and now has wizard problems.
Shadowheart : Has a mysterious backstory and a box, don't ask about it.
Astarion : Just got out of a really abusive relationship and has vampire problems.
Wyll : Is a charismatic fraud that plays hero and has warlock problems.
Lae'zel : Just wants to be told she's a very good girl by her queen so she can go and butcher things with a dragon.

The protagonist?

Tav : Is an eldritch loot gremlin who has absolute control over everything and is going to relive all of these stories indefinitely until such time as it has slept with/murdered all of the things.

The side characters.

Minthara : Is evil but and generates 50% of the thirst in this game.
Halsin : Is a druid and generates the other 50% of the thirst in this game.
A whole lot of tieflings : They are either refugees with refugee problems or just on the run from some thing or another.
Goblins, lots of goblins Found a new god and it's going about as well for everyone as you'd expect.
Volo : Is older than he looks and completely full of bologna, keep him away from your eyes....
Auntie Ethel : Is older than she looks and probably cooks children in a cauldron when she doesn't have them raw.
The tadpole : @#&*#@ %^ *&^ #$%^@#5% "?_+ =&% &($@ <@$
The one from your dreams : Is either out to kill you or completely on your side.
And all of the stuff in the underdark: not really worth coloring or mentioning really.


"Everyone is the hero of their own story."