Dr. Ellis is a mysterious therapist. Though he is only shown in person in A Cry From Winter, his character has a large impact, and creates connectivity between the stories and figures of the WinterVerse.
Little is known about Dr. Ellis's origins or his background. It is hinted that he was once a member of The Fearful Four, under the name "Deither Ellisviel" (or "Plague"). Indeed, the Doctor seems to be able to travel between timelines: he has been mentioned not only in A Cry From Winter, but also Michael's Camera, The Mayhem Theory, L Jameson, and many other blogs and vlogs, confirming that he's worked with patients across many timelines.
It is inferred that Dr. Ellis knew Marcus Winter, and may have worked with (or interned for) him at Institute 11. Ellis uses an experimental form of therapy including a new drug, which is still in its trial stages. It's unknown what exactly the drug is called or what it does; however, it has been shown to cause many of his human patients to see entities such as Lurkers, and occasionally undergo a ".relapse".
A Cry From Winter
Dr. Ellis is introduced in the first entry of A Cry From Winter ("A Bitter Winter"), where he can be seen talking to Alex about some issues he’s been going through. We learn that Ellis is the therapist overseeing Alex Winter's psychiatric needs after the Winter Family massacre, providing him with his court mandated therapy and starting him on a trial of his new medication. Alex later begins doing a video log under Ellis's orders.
Ellis next appears in "Another Session". Alex tells Ellis about having dreams about a "long nosed figure" (which Ellis suggests is simply a side effect of the medication). Ellis is later seen having a session with a girl in a white mask and a Ghostbusters shirt.
Ellis appears in "Several Months Prior", which takes place at a a therapy session several months before "A Bitter Winter". Ellis informs Alex that he will be starting him on a new, experimental medication. The camera then pans back, revealing a black Lurker mask hanging on Ellis's wall.
In "_", Ellis appears after Alex has takes an entire bottle of pills, causing him to have a .relapse (and to be attacked by a Lurker). Ellis shoots at the Lurker with his pistol and chases it off. He then returns to Alex (who is wounded on the floor), and proclaims that his father would be "very disappointed to see [him] like this"; he adds that he'll make sure that doesn't happen, before confiscating the camera. At this point, the timeline seemingly resets to an earlier point (though Alex still remembers the events that happened prior).