Kalen Montag was born on Coruscant during the later years of the Galactic Civil War, the son of a navy Captain and a wealthy socialite. As such, his childhood was a mix of privilege and discipline. With the fall of Coruscant to the Rebels, however, the Montag family was forced to relocate, leaving their old life, fortune, and status behind. Captain Montag was killed in battle, leaving only Kalen and his mother to continue the family line.
At eighteen, Kalen applied to the Imperial Academy, and was accepted, eventually earning a commission as an officer in the Imperial Navy. His first assignment was aboard the VSD Sovereign, a border patrol vessel, where he served as part of the ship's gunnery crew for two years. During his time on the Sovereign, Kalen was a part of several small-scale battles with New Republic forces, as well as numerous encounters with local pirates and criminals. By the age of twenty-two he had earned two citations for exemplary service, and risen to the rank of Lieutenant.
From the Sovereign, Lieutenant Montag was transferred to the ISD-II Ultimatum, a sector group flagship. There, he remained a part of the gunnery crew for another year, but soon found himself reassigned to the bridge as a tactical officer, the station at which he remained for the remainder of his career in the Imperial Navy.
As time passed, however, Kalen become more and more disenfranchised with the Empire. While he had been indoctrinated since birth with its principles, and most had stuck, he began to feel something fundamentally wrong with the side he had chosen. With the continued fragmentation of the Empire, Kalen took the opportunity to leave when he saw it. It was not long before he was banging on the New Republic's metaphorical front door, begging for a position in their fleet.
He lasted a full year aboard in the New Republic Navy before being handed his walking papers. Kalen simply didn't fit in, still hanging on to his old 'Imperial' tendencies, including a thinly-veiled intolerance for non-humans. While he was an excellent officer, performing his duties beyond reproach, his attitude was simply lacking.
When the opportunity to join the crew of the Corellian Banshee arose, Kalen jumped at it, hoping to redeem himself and prove himself a value. It's not exactly going well.