CC-7765, like most of his brothers were bred on Kamino.
He was trained as a Commando in the years prior to Geonosis. After the battle his squad was deployed in multiple missions, until a mission that nearly wiped out the entire squad.
Only 7765 and his brother 7766 made it out in one piece, 7764 was severely wounded and his brothers raided a civilian hospital to get him medical attention.
7764 started to show signs of dementia and violent behavior from his head injury which prompted the staff to perform a deep scan of his brain. Finding a suspicious object, the staff did a biopsy and discovered a biochip badly damaged.
7764 showed immediate signs of improvement after the biopsy.
The lead doctor questioned 7765 and 7766 about it, but the brothers had no knowledge of the chip. They both offered to be examined to see if they also had chips, in the hope of answers. After the scans, they found that they too had chips.
The doctor asked to extract them for study, the brothers agreed since they saw what happened if the chips went bad, they didn't want to be liabilities on the battlefield and risk the lives of their many brothers.
The staff managed to stabilize 7764 as the Droid Army began their final push into the city, overwhelming the battered Clone battalion stationed there.
7765 and 7766 decided to evac 7764 and as many hospital staff as they could and managed to save thirty eight. When they reached the safety of a Acclamator Assault ship, they realized the doctor was one of the many that were lost in the siege. Despite the fall of the city, 7765 and 7766 were commended for their actions and offered to be trained as Advanced Recon Commandos (ARC- Troopers) They agreed, and quietly sent 7764 to civilian recovery center, reporting 7764 KIA.
After completing their training, 7765 and 7766 were re-designated from RC to CC.