Chak grew up as a street urchin on Virgillia, orphaned at the tail end of the Clone Wars. He was consistently in trouble with local authorities. Usually for pick-pocketing and running scams. At the age of 18, he found himself absolutely desperate. As a last resort, he applied to join the Empire. Despite a rap sheet a mile long, the Empire accepted him. A very surprised Chak shipped off to become a stormtrooper.
After training, he was assigned to the Outer Rim, where he was frequently helping hunt Rebels and put down uprisings on ‘unruly' worlds. Chak started having conflicted feelings about what the Empire was doing. At the same time, a more base feeling enjoyed the power the stark white armor gave him. He wasn't starving, people didn't pick on him or shake him down. Hell, people were trying to bride him for once. Chak finally had a home with the Empire.
That familial feeling grew when he lost his right arm in an engagement with rebels. The Empire gave him a cybernetic arm. That made him feel more powerful and loved by his surrogate family. They would take care of him.
Chak's love of the Empire became a morale booster for his squad. Even after he proceeded to lose a leg to a rebel explosive. When they gave him a new leg, it only strengthened his loyalty. Same when he lost the other leg. Chak was practically fanatical at that point.
So when he was partially crushed by building debris and needed a cybernetic lung, rebuilt collarbone and half a rib cage, he was expecting the best from the Empire. Instead they patched him up as best they could. No cybernetics this time. Instead they discharged him, no longer fit to serve.
To add insult to injury, the Empire started to charge Chak for the cybernetics. He also couldn't keep his right arm on as he was missing quite a bit of bone and muscle to hold up his shoulder. Chak's faith in the Empire was shattered. He couldn't find much work that he was suited for and soon the Empire repossessed his arm and legs.
The whole thing crushed Chak. Who was once again living on the streets running scams like he did when he was a child trying to make money to pay the Empire's bill. Around the same time he made ‘friends' with a Rodian droid mechanic and rebel sympathizer. The Rodian wasn't fooled by the scams, but he saw an opportunity in Chak and offered him a job in his shop.
Chak started to learn the ins and outs of droid maintenance and in turn he and the Rodian started working on new legs from an old B-1 battle droid. While they worked on this, the Rodian also started introducing Chak to Rebel operatives, trying to persuade him to join to keep the Empire from doing to anyone else what they did to him.
Chak thought about this for some time, and finally agreed. The last thing he needed was something to help restructure the right half of his rib cage and support a prosthetic right arm. His Rodian friend helped him design and modify the armored carapace of a B2 super battle droid to reinforce his right side.
Once he was rebuilt, Chak left for a rebel cell to retrain while on the job. His new cell immediately started calling Chak ‘Clanker' for the amount of battle droid parts he has. Despite the ribbing, the droid parts came in handy on a number of occasions, especially the extra armor of the B2 battle droid parts. Between that and experience, Chak continued to survive through Hoth and Endor.