Unlock Sebulba:

Play as Anakin in tournament mode win every track
in first place.


Unlock Sebulba episode 1 pod: 

Play as Selbulba in tournament mode and finish every
race in first place.


Get Mawhonic:

Beat the last of the first four circuits in tournament
mode.

Watto:

Get in the fastest 3 lap times for every track to 
unlock Watto.


Darth Vader:

Unlock Anikin in episode one, Watto, and Darth Maul,
then win tournament mode as one of those racers to 
unlock Darth Vader.

Darth Maul:

Get the top KO score for every track to unlock Darth
Maul.

Race as Anakin in episode 1:

Get the fastest lap time for every track to unlock 
Anakin.