Hello again, I just saw on Kickstarter an answer from Larian to a question similar to what I asked so I'll post it here in case someone's interested :
Cr饌teur Larian Studios LLC
"@weresheep: in regards characters in Single Player mode, that's still being discussed. Of course we want to give people as much freedom as possible, but asking you to sit there and make four deep, complicated characters might be a bit much. There's a middle ground in there somewhere and, like many things, it's still under discussion.
But the thing that we want to remind people about in single player is that the companions aren't just henchmen that you pick up along the way. Each and every one of them will be just as deep and complicated a character as the character that you make when you start the game. It's going to be a great experience putting your party together, and we're really looking forward to seeing how people react to it!"
So I guess it's still undetermined.

If I may add my opinion : while I do appreciate Larian's work on creating fabulous companions with deep backstories and complex personality, I can't help remembering that in every RPG I've played, there always was companions whom I liked more than others. So, of course, we don't know yet how many available companions there will be at release but I have to say, and I'm sure I'm not alone, I wouldn't mind *at all* being able to create all of my four character and companions for the adventure.

If Larian decides it's too much, well, I can't blame them, I guess.