There are 6 total player to unlock:
- Hero unlocks Lucien
- Hammer unlocks Scythe
- Reaver unlocks Ben Finn
- Garth unlocks Maze
- Jack of Blades > Complete Dark Albion Credits level
- Sir Walter > Complete Albion Credits level
- Gabriel & Theresa > Play Fable: The Journey (Not necessary)
You must purchase 4 of the characters on the ability board, by landing on the green tile on the top of the board and purchasing them using 15,000 gold, with each puppet.
Note: It is not necessary for you to buy Fable: The Journey for this achievement, contrary to speculation.