Background
Hallex Garo is a member of the Rebel Alliance Intelligence Service, with several high-risk operations under his belt. Well versed in a mix of clandestine infiltration and undercover missions, Hallex routinely works to uncover and counter ISB operations across the galaxy.
Originally a skilled thief, Hallex regularly stole to provide for himself and his sister, Kara. When she fell sick, he discovered the medicine she needed was being rationed by the rebels, who were currently occupying the area. Determined to look after his sister, Hallex broke into the depot to get what he needed. When those meds ran out, he took more.
On his third trip back to the depot, he was arrested and faced a firing squad. The Zabrak in charge informed him that they had discovered his theft and had been waiting for him to return. She introduced herself as Zyrrah Sunlet, a liason for Rebel Intelligence, and offered him a job. She explained the complexities and conundrums of the mission, and offered his sister full medical care to sweeten the deal. Kara was immediately taken into a rebel hospital and treated, and Hallex was sent off on his first assignment.
He undertook several missions during her recovery, however Kara had found sympathy for the rebellion. As soon as she was discharged, she enlisted to fight the Empire, and Hallex was offered another choice. He could remain in service, and Rebel Intelligence would pull strings to keep Kara away from the front line, or he could leave, and she'd receive no special treatment. This deal has caused friction between Hallex and his sister, who resents him for interfering, however he maintains his belief that he's keeping her safe.
Motivation
Hallex's duty to the Rebellion serves his greater purpose of protecting his sister. This drives him to go to great lengths and accept personal risks to accomplish his objectives.