Star Wars: Shattered Peace
Ahsoka Tano didn't know what she was signing up for when she went out of her way to recruit the remaining free clones. Some of them were agreeable to their old general.
Others, one bar've in particular, have had enough of battle. He knows his duty, but he refuses.
And Rex wants to kill him.
This particular clone was also tortured on Kamino. On orders of Lord Vader. He's been made a Force-user. He trusts no one, likes no one, wants to be around no one, and is quite flippant to boot. Clones tend to have long memories and this one is no different. He believes that he's useless. A nothing. A traitor worthy only to die.
Ahsoka thinks he has potential. (He thinks the Jedi got hit on the head one too many times)
But to unlock it, this unlucky clone has to over come a host of brothers who want to kill him.
And, of course, the Empire wants their proto-Inquisitor back.