Black Knight – Nathan Garrett is unlocked on the third level "Castle Hassle" at the very beginning of the level as soon as you start.
Black Knight (Dane Whitman) – Head to the right side of the main gate area where you will find a small golden chest next to the two fires in a small alcove. Destroy this small chest then use TRACKING to follow the trail up and around the castle wall. Dig up what you track down to find this character card!
Black Knight – Nathan Garrett: Once you actually buy Dane Whitman, Nathan Garrett automatically unlocks, free of charge.
Once unlocked just defeat 30 enemies in free roam or free play with Black Knight – Nathan Garrett.