Atton: he is very hard to Stun, poison, or paralyze.
Bao-Dur: His arm (it's mechanical) allows him to 'bash' down certain force field doors quickly. His robot (think of those little floating orbs Luke practiced against on the Falcon in episode 4) will help you in combat by shooting enemies or repairing friendly droids. He upgrades it over the course of the game. Either he or his droid function as the 2nd half of the mobile workbench.
Kreia: she gives everyone an earned experience bonus. Even better, the unique bond she shares with the player allows him to Force Chain attacks and defense. Any Force Power cast on Kreia or the player will be automatically applied to the other party members as well. The exp bonus works like this: It's a percentage. The amount of the percentage goes up as Kreia goes up in level. The stages are 3%, 5%, 7%, the levels are at 1, 7, 13.