Seamus Jackson was the son of renowned Marine Corps NCO Dean Jackson and Kathryn Miller, daughter to the then-governor of Aphrodite II. He eventually became a Marine like his father, some time after advancing to the ranks of the ODSTs. Years into the Human-Covenant War, he ended up in the employ of the Office of Naval Intelligence, Section Zero. Long after the great war, he was assigned to Lambda Company of the SPARTAN-IV Program.