Characters
Kreia
Kreia is the first character you encounter in the game, when you find yourself confused and alone on Peragus. She's an (ex-?) Jedi strong in the force.
You seem to have a strange link with Kreia, although she's a little tough to understand, and her motives aren't clear. She's a little like the Knights of the Old Republic II version of Bastila from the first game — only she seems a little darker to start.
As your mentor, Kreia increases the amount of experience points you earn while she's in the party. Her force chain ability also allows force powers used on her to affect you as well. She's also immune to your influence, meaning that her "Neutral" status remains unchanging.
Atton Rand
If Kreia is KOTOR II's version of Bastila, then Atton is the new Carth — at least at first. He's a bit of a mystery too, and often has trouble figuring out why he follows you around.
Atton begins as a scoundrel, but as the game advances he could become a Jedi Sentinel. This only happens on Nar Shadda after you talk to people in the refugee sector. Confront Atton about what you've learned, and you can change him into a Jedi if you have enough influence with him.
T3-M4
T3-M4 is first shown at the beginning of the game, and it becomes instrumental in your escape from Peragus. As a droid, T3-M4 is quite neutral. This droid is very useful at providing computer spikes, though.
Bao-Dur
Bao-Dur is an old friend of yours from the Mandalonian wars, and he frequently refers to you as "General".
Like Atton, Bao-Dur can also be switched over to a Jedi class (Guardian). You need to build up some influence with him, until one of your conversations has him saying that "Having you here has an effect on me, General." Keep talking until he agrees to let you train him.
Handmaiden
The Handmaiden is found in the hidden base at the end of your escape from Telos. She's all about the light side, but seems pretty willing to abandon it for you. She has some connection to the force, and can become a Jedi Guardian with influence and a few sparring matches. You'll need to talk to Kreia about the Handmaiden's past, too.
The Handmaiden is only around with a male as the main character.
Disciple
The Disciple is found in the underground level of the Enclave on Dantooine. He's a young Jedi hopeful, with a very simplistic view of the world and a love for the Republic.
The Disciple is only available when the main character is female. The disciple is the equivalent of the Handmaiden for female characters, except that he becomes a Jedi Counsular.
Visas Marr
Visas is a Sith apprentice to Darth Nihilous. She appears in a cutscene with her Sith master, and then on the Ebon Hawk the next time you re-enter the ship. She initially attacks you, but after you put her in her place she proclaims herself to be your servant.
Mandalore
Mandalore is the head of the Mandalorians (or what's left of them). You first meet him on Dxun (a moon of Onderon), where he helps you get to Onderon, and later joins your party permenantly.
G0-T0
The G0-T0 droid is made yours when you leave Nar Shadda after you leave Goto's Yacht. The droid is somewhat forced on you by Goto. Its mission is an uncertainty.
Mira
Mira is a bounty hunter, who you encounter on Nar Shadda. You can only get Mira if you're on the light side, or neutral — and getting her means you can't get Hanharr.
Hanharr
Hanharr is a Wookie bounty hunter who you find on Nar Shadda. Getting him requires a dark-side orientation, and rules out your option to get Mira in your party.
HK-47
HK-47 is a remenant from Revan's time on the Ebon Hawk (from the first Knights of the Old Republic). He's in shambles when you discover him, and requires that you find 4 missing HK components throughout the game and repair him.