Background
"Born" from a vat as a clone trooper, R8T-9793 he entered service in the army led by Mace Windu on Geonosis. A typical soldier he performed well but not with unusual honors throughout the Geonosis campaign as his squad's scout.
His platoon was sent towards the end of the Clone Wars to secure a collection of force relics collected by the Separatists. As he was patrolling the perimeter, he was shot by bowcaster bolt in the back of his head; left for dead among the rest of his squad, all he remembers of his assailant is a elaborately carved bowcaster, marred by old burn marks.
He survived but came back to service changed... deeply religious about the force and considered to be erratic by his stoic fellow troopers. He became known for daring yet risky maneuvers in combat, bucking established tactics, as well as irrational emotional reactions to discussions of Jedi and Sith. The bolt causing all this still remains, as fragmentation prevented safe removal. It sits, barely visible, at the base of his skull in the back.
After he returned to combat, when Order 66 went out he wasn't affected, as the bolt had disrupted his inhibitor chip. After seeing one Jedi murdered by his company, he knew he that protecting the balance required trying to warn the others on the planet. A single trooper wasn't able to change much except for buying time for an unknown padawan to escape.
Having left his company, he was determined to have deserted and is sought by the Republic / Empire. In the years before the rebellion, he joined the Church of the Force and wandered the galaxy, becoming a collector of minor force relics and Jedi trinkets, returning many to their original planets. It was this, and the destruction of Jedha, that led him into contact with the nascent Rebel Alliance.
Now old from accelerated aging, his military experience lets him pass as a stormtrooper and or his occasional fellow retired clone trooper... as long as he can hide the the bolt at the base of his skull.
Motivation