Shadow Freddy is a secret antagonist in Five Nights At Freddy's 2 and the tertiary, later true secondary antagonist of Five Nights at Freddy's 3. He is (much like Shadow Bonnie) an enigmatic character in the second game who's intentions are unclear whereas in the third game he is clearly seen helping the Killer destroy the animatronics.
Role in the game
In Five Night At Freddy's 2, the player may rarely encounter what seems to be a shadow or a dark model of Freddy Fazbear. It appears in Parts/Service, sitting in the location old Bonnie normally sits. He will shut the game off should the player observe him for too long (without a jumpscare), much similar to Golden Freddy's role in the first game or Nightmare's effect in the fourth.
It appears to be in a slumped position, similar to that of Golden Freddy's. It also uses Golden Freddy's model. Brightening up this image reveals that this hallucination is purple in color as with several other key items/characters in the franchise, bearing comparisons to the purple figure from the secret mini-games, the Marionette's "tears," Fredbear's hat and bowtie among other things.
In Five Nights at Freddy's 3, within the in-between night mini-games, an animatronic bearing heavy resemblance to Shadow Freddy is used by the purple figure and utters the phrase "follow me," to lure the animatronics to a secluded spot, so that he may ambush and dismantle them. While the Freddy suit is recognized as purple due to its association with the purple figure, its purple coloration and the darker color of its hat and bowtie lends credit to the theory this particular animatronic is indeed Shadow Freddy.