I thought about a native inhabitant of the region, maybe a druid or a barbarian...but I decided that I didn't really want to have split loyalties between the party and the natives.
Concept 1:
Fighter/Rogue
I'm currently leaning toward a concept of one of the keep's few remaining original garrison. Maybe he and a few of his men have a camp in the woods (if the keep is still held by nefarious forces) Or he and a few others are all that remain, desperately hoping from some kind of relief from the motherland that never came. He was a bandit in his early life, but was caught and brought to trial by the constabulary. Rather then be executed, he was given the options to "take the black" (pardon the Game of Thrones reference) and was sent to the outpost as a conscript.
He found that the isolated life away from the city agreed with him, and found a peace that he had never known. When the keep fell, he saw what little happiness he had disappear. He is bitter and disillusioned, but he is also driven to see the keep restored and return to his military life.
Concept 2:
Bard Spy
Lord Dulat's master of spies who managed to slip Bastion Arcata's location and helped arrange for her escape when her lord father's estate was overrun. Since he was well known in the realm as a courtier of Lord Dulat, his only option after Dulat's fall was too join Lady Arcata and Bastion as they fled to the Keep on the Border Lands.
He is motivated by a sense of immense guilt for allowing Lord Dulat to be murdered on his watch and feels it is his duty to watch over the only remaining heir to the House of Dulat. He seeks to restore the family to their former place in the kingdom, as well as punish those who conspired against the Dulats.