As you earn money, you can buy cool new characters from the main bar area:

gonk droid 250
pk droid 350
battle droid security 300
battle droid 200
battle droid commander 1000
droideka 10000
royal guard 800
padme 800
darth maul 15000
clone 2000
geonosian 2000
battle droid geonoiss 300
supper battle droid 5000
jango fett 65000
boba fett 800
luminara 20000
ki-adi mundi 25000
kit fisto 35000
shaak ti 15000
count dooku 45000
grievous bodyguard 30000
general grievous 200,000
dooku 45000
clone III 600
clone II pilot 700
clone ii swamp 800
clone ii walker 2500
mace windu iii 30000
disguised clone 2750
darth sidiuos 150000

still 2 more hidden

Main Star Wars Lego Walkthrough