Windom Earle is a former FBI agent, who was once partner and best friend to Agent Dale Cooper. He is played by Kenneth Welsh and features in the latter half of the second season as the primary antagonist following the conclusion of the Laura Palmer investigation. He is an evil genius and a master of disguise, well-versed in esoterica from all parts of the world. He has extensive knowledge of the “dugpas”, ancient sorcerers from Tibet dedicated to pure evil. Cooper says of Windom Earle: “his mind is like a diamond: cold, hard and brilliant.”
Earle was institutionalized after killing his wife Caroline, the love of Cooper’s life, and trying to kill Cooper as well. He escapes and comes to Twin Peaks, hiding out in the woods, with the announced intention of ruining Cooper's life before killing him. He captures and enslaves Leo Johnson to act as an unwilling henchman, and finally kidnaps Annie Blackburn to lure Cooper into a trap.
He has a fascination with the Black Lodge, whose secrets he is trying to unlock, as well as black magic. Major Garland Briggs states that Earle was involved in Project Blue Book, as was Briggs; however, their investigation in which Earle was involved was directly related to Twin Peaks and not the usual UFO investigations; his growing obsession with the Lodge led to his dismissal from the project.
He is obsessed with chess and this plays a major role in most of his first appearances, when he decides to use real people as the pieces in a game against Cooper. The true intention of this is to make his way into the Black Lodge, whose power he hopes to harness, while also getting revenge on Cooper. By claiming to have control over Annie's survival, Earle attempts to blackmail Cooper into offering Earle his soul and stabs him in the Lodge, but BOB, who says that Earle cannot ask for souls, turns back time to stop this and presumably takes Earle's soul for himself.
Dramatically, Earle represents the dark counterpoint of Cooper: an FBI agent, once pure and following a code representing good, ultimately being corrupted and pursuing evil; the implication being that even a pure soul like Cooper can wind up like Earle, as the Cooper doppelgänger represents.
Read more about this topic: List Of Twin Peaks Characters, Outsiders