Mr. Scratch is a doppleganger of Alan Wake. He first appeared at the end of Alan Wake and it is implied he was created by Thomas Zane, possibly as part of the balance. However in Alan Wake's American Nightmare Mr. Scratch states that after Alan disappeared all that was left were the wild stories about him allowing Cauldron Lake's ability to use art to shape reality to make the stories true, thus creating Mr. Scratch; Alan Wake as the stories portray him. The true purpose of Mr. Scratch is unknown, but it is implied that he returned to the surface of Bright Falls while Alan Wake was writing the ending to Departure at the bottom of Cauldron Lake. It is probable that Mr. Scratch was supposed to return to Barry Wheeler, Sarah Breaker, Cynthia Weaver, and Alice Wake, but his reasons and purpose are still unknown. Whenever a character refers to Mr. Scratch, the sound of the word "Scratch" is muffled out, replaced by either static or, appropriately, a record scratch.

(Taken from the Alan Wake Wiki's Article)