Alan Breck, or Alan Breck Stewart, to give him his full name, is a member (appropriately enough) of the Stewart clan. Alan is not really a dynamic character in the story. He exists rather as a helper to David, a loyal assistant who will guide him (and us) through the strange, exotic world of the Scottish Highlands. Stevenson also uses him to provide a touch of dash and color to the story, in keeping with its status as a rollicking tale of high adventure.
Clan is of huge importance to Alan; and he is fiercely loyal to the Stewarts, ready to defend their honor with sword or musket at the drop of a hat. Alan provides a neat contrast to David, in that David is less worldly and also much less grounded. Alan's close ties to his land, his clan, and his religion emphasize the great importance of loyalty, one of the book's most notable themes.