Karlach is a potential positive companion, you meet her near Waukeen's Rest when dealing with the Zarielite ex-paladins of Tyr. The datamine showed that the other companions are Minsc from BG1 and a halfling werewolf named Helia. This makes eight companions, as many as Larian has said they have worked on so far. I cannot say for sure what the leaked companions' personalities are, but if there are any positive companions, they must be in there somewhere.
