Answer to this question mentions Mr. Scratch. I do not remember the name mentioned in the game, but I must have missed it.
Can someone please remind me?
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[SPOILER ALERT] (Please add spoiler indication to the question title.)
This site details the answer.
Here's the relevent text:
Alan fights his way all across the town, fighting and escaping from an army of Taken and Poltergeists sent by the Dark Presence to stop him. When Alan arrives at Cauldron Lake, he dives in to save Alice. But as soon as he does so, he blacks out again. When he awakes, he is at the bottom of the lake. The Dark Presence tries stopping him from finding the Clicker by using hallucinations and tricks to get Alan to give up. But Alan finds the Clicker and uses it to drive the Darkness and hallucinations away. Alan is visited by Thomas Zane in a diver's suit, who tells Alan that Barbara Jagger has no heart, he must "fill it's heart with light." Before Alan leaves to find Jagger, Zane introduces Alan to Mr. Scratch. Mr. Scratch is a clone of Alan Wake who will return to the world while Alan writes the ending to the book. Zane leaves and Alan finds Bird Leg Cabin under the lake.
According to this site. He was first mentioned in Episode 6: Departure after Alan destroys the tornado.