Marcus was a very cold man, as symbolized by his icy blue eyes. He had an intense loyalty to his people, for whom he would do anything to restore to their former glory, although he would not hesitate in expiring any Terran that displeased him. He had no time for incompetence from any of his subordinates, to whom he was known to deal out sometimes excessive force if anyone erred or insulted him.
He also had a sometimes psychotic hatred of Cardassians, more so than the average Terran, although no one was exactly sure where this intense dislike came from, as it seemed to go deeper than simply the fact that they along with the Klingons enslaved him and his people.
Despite these personality quirks, Marcus was an exceptional engineer, and had a penchant for creating enhancements to weapon systems, either hand-held or ship-mounted, as he demonstrated when he once somehow overloaded a stolen Cardassian disruptor that vaporized the Cardassian holding it, as well as five of the alien's squadmates.
He also had an extensive understanding of warp drive, and was committed to finding a speed faster than warp 9 that would enable the rebellion to gain a new advantage over the Klingon-Cardassian Alliance and re-establish the Empire to its former glory.
Not much is known about Marcus' history, except that he was used as a weapons engineering slave for the Alliance from a very young age, which explained his incredible understanding of weaponry. It was also there that his reputation for overly excessive punishment for incompetence was established.
After his escape from slavery, Marcus found his way to the captured Alliance station Terok Nor, where he heard of a young human capturing an alien ship, naming it after the old Imperial Starfleet ships of the original Terran Empire, which greatly interested him.
- Marcus joined the Descent on Delta Vega after the ship crash-landed on the planet.