Background
Having served in various military positions as a medic, Mayellen knows this civil war WILL cause death, it's her job to limit the dying to the Empire as best she can, ad is not afraid to shoot first if it means saving lives. Her family were famously pacifist doctors who treated both sides, but Mayellen did not harbor such feelings of peace, getting into many fights as a child. These fights landed her in deep trouble numerous times as she fought with anyone who teased her or disagreed with her or her parents views, child, adult, Imperial, law official, it didn't matter, she loved a fight, even if she couldn't possibly win. Despite this, she became rather good at practicing medicine, and was often healing those she had fought, which was intended as a punishment but she took it as prepping that guy for the next fight. She was in an imperial prison when the Rebellion approached her, it was her fellow inmate, a Torgruta by the name of Abraari To, a commander of a starfighter wing downed over some distant planet she never heard of, but it offered a way to practice both her medicinal practice, but it also allowed her to fight the Imperials as she pleased. A rebel attack to get the commander was her escape route, and the Rebellion, her call to usefulness.
Motivation
Fighting the Empire, healing her allies