Here's some ideas, Tuned-Out (and others), for some characters that might work for this adventure. Enjoy!
A human swashbuckler, a mariner by trade from Zazesspur, looking for "safer" employment than the dangers presented from life at sea. S/he is an expert swordsman, skilled with a rapier and dagger (or dual short swords).
A human (or elven) druid from Mosstone, who serves the Druids of the Grove. S/he has been assigned to this band of adventurers by the Druids out of concern for the people and to protect the interests of the Druids. A badger, eagle, owl, or wolf serves as his/her companion.
A human cleric or paladin of Ilmater or Tyr, sent by his/her local congregation to help those living out of the fringe. Primarily concerned for the welfare of the people, s/he sees pushing out the encroaching monsters and brigands as a way to pursue his/her main goal.
A human paladin or cleric (or fighter, I suppose) of Helm or Torm, attached to this group of adventurers because the goals of his/her diety and church line up with the goals of this adventure -- defend the line! Vanquishing evil and protecting the people is at the center of what this person is all about. Depending on the type of adventure, perhaps focusing on mounted combat as one of Tethyr's newest knights. (Check with the DM... if the adventure is going to be mostly underground or center around areas where your mount won't be of much use, focusing on mounted combat won't be much fun.)
A human or dwarven (or halfling) craftsman/merchant (rogue), with skills in a craft (woodworking and locksmithing to make the cool chest), open locks and disable devises, diplomacy and bluffing, and maybe some athletic skills (climbing, jumping, riding).
A stout dwarven mercenary (maybe a fighter, maybe a berserking "barbarian") from the Starspire Mountains. With the civil war over, he's just looking for work and some heads to lop off.
A profit minded halfling (or human) adventurer (rogue or beguiler) who sees this adventure as a way to make some real coin... who knows, maybe there is unclaimed treasure to be found amongst these long forsaken keeps and forts?
A human wizard (or sorcerer) from Darromar, sent out as one of the Queen's representatives (or representing the interests of one of the colleges in the city). S/he is tasked to make a careful search for a specific item believed to be lost somewhere near the area in which the PCs will be adventuring. (check with the DM about this one as it will require the DM to somehow tie it in to their plans for the game.)