Linnie is a side character who has yet to make a confirmed appearance on camera. The first mention of Linnie occured in the video "Reunion.". Her name was the signature on note found by Jeff, Evan and Vince in the burned out shell of Jeff's school. The connection was very quickly made to a letter mentioned in the Rake creepypasta, which was also addressed to Linnie.
Later, during the events shown in Cops Checked, No Body, a note was found clutched in the hand of the Dead Girl that was revealed to have been written by Linnie. That letter (the contents of which were finally revealed to the audience in Ashen Waste) detailed Linnie's intention to travel east and contact the EMH crew. It mentions specifically that she had slept the night before in the Logan Inn, a historic inn in Bucks County, Pennsylvania. It has been speculated that the Dead Girl is herself Linnie, but this has not been confirmed. In the Corenthal Journal Dated 11-12-1990 Dr. Corenthal recounts a dream that he had about Linnie's death. The circumstances of the death in the dream match closely to the death of The Dead Girl, but the time frame is off by 20 years.
A set of coordinates were left in Alex's closet (shown in the video Business Cards on a Plane) that led to a bag in the woods near Jim Thorpe, PA. Inside that bag was yet another letter addressed to Linnie, signed by William, a name also mentioned in the original Rake story. Jim Thorpe is approximately an hour north of the Logan Inn. These two facts together lead many to believe that the bag belonged to Linnie.
The Corenthal Letter dated 10-27-1981 reveals that a Linnie was a patient of Dr. James Corenthal, taken in sometime after he and his wife had adopted the Mining Town Four. Unlike the other Fairmount children, she didn't come from Centralia (the letter mentions "horrible New York attacks") and doesn't seem to display the same sort of symptoms as the others (her "personal demon" is "more feral", according to the letter). The letter also mentions that Corenthal is taking her east in an attempt to save her from the fate of the Mining Town Four, who have "sucummed". A brief mention in the Corenthal Letters dated 7-22-1995 and 7-23-1995 indicates that she managed to escape whatever happened to the other Fairmount children.
Whether this Linnie is the writer of these letters or another iteration (see the Iteration Theory) is unknown at this point.
The reasons for Linnie's quest to find the EMH crew, an explanation for her name being on a 40 year old letter in the original Rake story, her current whereabouts, and whether or not she is alive are all subjects of speculation.