The character's backstory is given as having been born on Mars Colony. In an early episode of season one, Sinclair states that his family had been pilots "ever since the Battle of Britain" and Sinclair's father was a fighter pilot for EarthForce who participated in the Battle of Balos, the last engagement of the Dilgar Invasion. Sinclair continued the tradition and became a fighter pilot. Sinclair enlisted in EarthForce in 2237. During his time at EarthForce Academy, he met Catherine Sakai, with whom he had a relationship. After a year of living together, the two of them broke up, but continued to see each other off and on through 2258, when they became engaged. Sakai went missing in late 2259, while on a mission for the Rangers. The season one episode "By Any Means Necessary" establishes that Sinclair received Jesuit education as a young man.