- AHEM!
"Zach is Francis York Morgan's closest and most reliable friend, despite the fact that he is seemingly all in York's head. York constantly talks out loud to his "invisible friend" during his asides and reviews, as well as chatting with him idly during their off time. While Zach is never heard responding, York apparently does hear his replies. Zach is always there to help in any way he possibly can, often being the one that York goes over evidence with.
It is clear that Zach is not a physical manifestation (at least not one that the player can see) but it is initially unclear whether Zach is some kind of psychosis, a Twin Peaks reference (as Dale Cooper narrates his life into his beloved tape recorder, Diane) or just a narrative device for the story. Eventually, Zach's true nature is revealed.
Zach and York have been friends ever since a particular disturbing incident during York's childhood, when Zach came to console the grieving York who had witnessed the murder of his mother at the hands of his father. Ever since that day, Zach had always been there for York, helping him and keeping him company, and the two shared such a close bond they became the best of friends, despite the bizarreness of their relationship. York continued to keep Zach close to him into his days as an agent, where Zach was more than willing to help York on all of his investigations.
Towards the end of the game, Zach and York find out the true mastermind behind the events that shattered their childhood, Forrest Kaysen, a demonic entity that not only forced York's father to kill his mother to spare her from the red tree growing within her, but he also was behind George's(The Raincoat killer) descent into madness. This revelation also comes with the startling realization that York was the imaginary friend, not Zach.
York approached Zach in his time of need, swapping places with him and allowing Zach to recover in the Red Room locked deep within his mind. As they grew, they both forgot this detail, and Zach was more than happy to spend his days within the confines of their psyche. However, after this realization, Zach finds himself forcefully put back in charge of his body, with York resuming the supporting role."
There, I just ruined the ending of DP
edited 5th Jan '13 9:58:05 PM by eldritchseer

Oh, and who is Zach?