Background
One of the last generations of Kaminoan clones, Tagger was wounded on his first combat mission on the planet Akiva in the Outer Rim. This took place towards the end of the Clone Wars.
His medical transport was hit by Separatist ships, and got flung into space. Drifted there for 30 years, with Tagger in stasis in his bacta tank. Was picked up by scavengers, who woke Tagger up.
Motivation
Tech: Tagger's Clone Was blaster carbine is very precious to him, as he has been able to hold on to it since his training on Kamino. It is one of his very few links to his own "era." He calls his DC-15S carbine "Lullaby."
Belief: The Republic - Tagger believed in the democracy and honor of the Galactic Republic, and abhors what the Empire has become. He has therefore joined up with the Rebel Alliance.