Do you mind telling me?
Clearly i didnt know.
Mercenaries are usually characters fully created by the player. Name, sex, class and so on... you create them in the character creator, exactly as your main character.
They don't have anything to say and don't have any story. They're just bland character you can "recruit" to create a fully or partially custom team.
I can't be sure but I guess mercenaries in BG3 will be the same characters we had when we were able to create our full team through multiplayer.
Tadpoled + "a few things to say".