Filled with civic duty by parents. Family liked helping people. Studious kid and adult. Wanted to lend a hand to the republic and joined a relief organization. Found himself flying all over the galaxy delivering medical supplies and food to various disasters.
Over the years, he discovered he was a good leader and was often coordinating deliveries, schedules, and putting out logistical fires. Sean jokingly blamed his parents for having him around all that governmental red tape as a child.
During the Clone Wars, the relief org was busier than ever, helping war torn worlds. He was thrust into captain-hood when the bulk freighter he was on ran afoul of a Seperatist minefield. A blast killed the bridge crew. In the chaos, Sean did what he knew best: Calmly tell people where they'd be most useful and what to do. They listened and managed to save the ship.
It didn't take long for Sean's heroism to reach his higher-ups. He was promoted to captain and given his own relief ship. As the war dragged on, he started to get a bit cynical about the conflict. Both sides said they wanted peace and freedom, but Sean had seen so much destruction. He and his family were concerned about the direction the Republic was taking. Those fears came true with the rise of the Empire.
The final straw for Sean was when the Empire started intercepting and seizing relief supplies. He and some of his crew no longer believed that the relief organization would be allowed to do their work without the Empire intervening. They were right. The Empire had very little interest in humanitarian aid to war-torn worlds and moved to shut the organization down.
He and a few others absconded with a couple of light freighters from the aid organization and started smuggling in lieu of their previously official work. After they ran a few blockades and made the Imperials look like fools, they started making contact with other rebels.
In time, Sean learned his older brother and younger sister had also joined the Rebellion. Word got around regarding Sean's abilities and he was put in the fledgling Alliance navy. Initially he continued to captain the light freighter he'd technically stolen, but after Yavin was given the chance again to captain a larger ship. This time, something fast with more teeth: A Corellian Corvette.
At Hoth, Sean lost his brother to the ground fighting during the evacuation of Echo Base. Sean feared for his sister's well being and tried his best to convince her to get out and go home. Much to his frustration, she wouldn't leave. So he decided to fight harder.
He fought like hell from then on and at Endor, he pushed his crew and the corvette beyond anything imaginable. They survived, but the ship was ready to fall apart. He and the crew abandoned the ship to the salvage crews.
The newly formed Republic had a seasoned captain and crew without a ship. They also were forming a new battle group that needed the veteran crew. Sean and his crew found themselves on a new CR90: The Cardinal.