there are 6 companion characters (that you can also play as if you choose adding more story)
lohse, fane, beast, red prince, sebille, and ifan
you can play as all of them as well as make a custom character you can recruit all of them (up to a max party size of 4)
certain characters bring very very very bad events upon your group if you invite them (wont say who but its pretty obvious and you are even warned at some point during act 1)
you can start with 4 custom characters if you sorta cheat the game and invite yourself to the game 4 times on 4 different launches but it really doesnt matter that much since you get to pick their class for them when you invite them.