Here comes the big spoiler - you can also turn Atton into a Jedi on Nar Shaddaa! Make sure your influence is already high with him, you can do this in a few different places on Nar Shaddaa. Finally, depending on whether you are going light side or dark side, you get Mira or Hanharr respectively. He is probably one of the best ranged attackers in the game and has a ton of great dialogue. You are able to fully complete HK on Nar Shadda as well. While this happens regardless of the planet you pick, we are trying to maximize the number of companions you have. Upon returning to the Ebon Hawk for the first time you can encounter Visas and add her to your party.
You can also add two to three more companions to your party. On Nar Shaddaa the main character dukes it out with the Exchange. There are a few spoilers ahead so consider yourself warned. Star Wars Knights of the Old Republic 2 features a ton of freedom when it comes to player choices.