I am always interested in Military themed Role Plays.
I wrote a book last year about a Private Military Corporation (PMC) set in the 24th century, similar to the universe you are operating in, but decidedly NOT a Dirty Dozen type. I would urge you to alter the names of your two warring factions from Confederates and Union. It sounds too much like the US Civil War. I would offer suggestions if you are open.

My guy would be a human soldier who happened to be down on his luck; possibly possessing either sociopathic tendencies or maybe just go all out "Maggot" (Telly Savalas) on the group.