In Naruto Shippuden: Kizuna Drive for the PSP, there are several characters you can unlock to play with.
Here is the list of unlocakble characters, and what you need to do to unlock them:
Choji – Complete Chapter 3
Hinata – Complete Chapter 5
Ino – Complete Chapter 3
Itachi – Complete the SS rank mission where you Fight Shukaku then The Nine Tails
Jugo – Complete Scenario mode
Karin – Complete Scenario mode
Neji – Complete Chapter 5
Rock Lee – Complete Chapter 5
Sai – Complete Chapter 2
Sasuke – Complete Scenario mode
Suigetsu – Complete Scenario mode
Yamato – Complete Chapter 3