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.
In 2596, the in ONI Section 0 and 3 well known SPARTAN-144 "Leonid", was selected as a trainer for the SPARTAN-IV Program. However, there were many amongst Section 0's brass that disliked his sudden declassification, and those officers banded together, secretly assigning Seamus to keep an eye on him and to ensure that vital section 0 information remained secret; this was arranged by assigning him to serve as one of Lambda Company's co-operations chiefs, under the alias of Sergeant Major Charles Vale.
Though initially he dutifully followed his mission to the letter, over time while serving under Leonid's command, he came to respect the Spartan for his skills, which far exceeded his own. In addition, he started to soften as he trained the Lambda Commpany trainees, slowly coming out of his unemotional shell. Lately, this has manifested itself in him developing feelings of guilt for deceiving all his new comrades, since he is technically there simply to spy on them.