He's been living in the hills and woods for about three years, living off the land and successfully avoiding hostilities with others in the area. His secret backstory is an escape off a merchant ship that devolved into violent piracy. His strong moral code was not popular with the crew, and the bloody ship still sails the seas.
Details to come but important skills immediately are tracking, hunting, and campcraft, honed over the past three years. His family had a tradition of naval service, but McGuinn chafed under the rigid structure and served a single tour of duty before signing on with a merchant ship. The traveling life agreed with him until his last ship, on which there was a mutiny so that the first mate and his men could plunder other ships and isolated towns with no regard to life. McGuinn was to be put to death as part of the mutiny, but escaped, made it to shore, and drove inland.
He's not apt to say much, and is distrustful of people at this point. His peace after the bloodshed of mutiny has come from the solitude in nature and his meditations there.