I know there is a theory he is one of the sellwords Littlefinger takes on at the end of aDwD, Ser Morgarth, but I think that's a stretch. Instead, I believe he comes from someplace more familiar to us, from House Darry. Brienne and her little group reach the Quiet Isle led by Septon Meribald. Interestingly, they get to the Quiet Isle by walking across the mud flats at the mouth of the Trident at low tide, following a complicated pattern of going one way, then doubling back, then goin