The list is now complete! Thanks to everyone for their help:
Each character appears on the Abilities Board, on the Hero Tile. Depending on the character you're playing with at the time, the character you can unlock via this tile will change. (Thanks to Salvatore 2 for this info!)
Each character costs 15,000 gold
Hero unlocks Lucien
Garth unlocks Maze
Hammer unlocks Scythe
Reaver unlocks Ben Finn
Completing all default levels unlocks Sir Walter
Completing all dark levels unlocks Jack of Blades
# Dark levels are available after seeing the credits for the first time.
Theresa and Gabriel can ONLY be unlocked if you play Fable: The Journey, but they do not count towards this achievement (thanks Murtland!)
NOTE: If you're playing with 3 AI partners, you can use them to unlock their corresponding characters. Great way to speed this process up rather than with 3 real people who will only unlock the characters for themselves.