Of all the characters in Twin Peaks, Diane’s following is among the strangest - largely because, until the most recent episode of the revival, she had never been seen. One of the greatest ongoing mysteries of Twin Peaks was revealed to be Laura Dern in a silver wig, because, of course—what show did you think you were watching? While the series has never been big on provided solid answers to its mysteries, the fact that Diane was finally brought into this series is no small development.
There were rumors that had Twin Peaks been renewed for a third season in 1991, Diane would have played a major role in getting Cooper out of the Black Lodge (as the rumor went, she would have been played by David Lynch’s then-wife Isabella Rossellini). Here we are now, and we’re poised to see this story play out. With Cooper’s doppelganger incarcerated and Gordon and Albert beginning to understand that this leathery man with the dead eyes isn’t the man they remembered, it’s Diane the two call in to help with the investigation and confirm their fears. Her importance is underlined in The Return, Part 4.
Gordon: “Albert, before we do anything else, we need one certain person to take a look at Cooper.”
Albert: “I’m right with you.”
Gordon: “Do you know where she lives?”
Albert: “I know where she drinks.”
In the pilot, we met Dale Cooper and Diane simultaneously; Coop, the perky and optimistic young FBI agent, was recording a message to her, as he was wont to do. It was clear that they had a personal connection, as Cooper would go off on tangents and speak offhandedly to Diane, who was up-to-date with his history. The early scenes of Cooper recording these messages to her were important in establishing his character and his train of thought. While he (for the most part) maintained a professional veneer with Albert Rosenfield and Gordon Cole, and while he became good friends with Hawk and Harry Truman, he was at his most honest with Diane.
In the second half of season 2, we meet the new series antagonist, Windom Earle (or Owl in Dream, as some of us have come to know him). We’re given the backstory over time: Earle and Cooper were partners,and the FBI was getting wise to Earle’s increasing corruption and insanity. His wife, Caroline, was going to testify against him, and Cooper was supposed to protect her. They fell in love. One night, after having sex, when Cooper was vulnerable and distracted, Earle snuck in and murdered Caroline. The result left Cooper gravely damaged. In rare moments, Cooper’s guard would fall, and that plucky young agent would get mournful and quiet as he recounted snippets of this story to Harry and Hawk. There are few people who know the whole truth about the story and given Coop’s predilection of detailing the intimate along with the minutiae in his recordings to Diane it’s likely she knows his mindset better than anyone.
Diane’s mythic importance to fans also plays a role in this. By having her appear it also closes the case on a long-running fan theory: that there never was a Diane. The theory goes that since we never saw Cooper send these tapes out, and that his requests of her were fulfilled so quickly, that Diane never existed; rather, she was a therapeutic exercise for Cooper following the ordeal with Caroline - a way for Cooper to organize his thoughts and sort his emotions. In, Fire Walk With Me: The Missing Pieces, there’s a scene that has Cooper standing outside an office in the FBI Philadelphia office, talking to a person whose replies only he hears. This person, as he says, is Diane, though we’re never given any indication there’s anybody there.
Now, while we don’t know why we never heard her responses, we do know that there actually was a Diane. It also makes this following line from the audiobook “Diane...” - The Twin Peaks Tapes of Agent Cooper a little sadder and sweet: "Diane, I hope that you will not mind that I address these tapes to you even when it is clear that I am talking to myself. The knowledge that someone of your insight is standing behind me is comforting." While it is a risk bringing Diane in, it further shows how few people truly knew Cooper and how that small circle are still devoted to him twenty-five years later. If anything, Diane could very well be an audience surrogate.
We do not yet have context on what happened to Diane in the intervening years between season 2 and The Return, but it's interesting to note that David Lynch and Mark Frost have gone out of their way to establish that Diane has a drinking habit. There are two possible reasons to do this. First, it could be a red herring. After all, earlier in this miniseries, we saw Sarah Palmer chain smoking and drinking alone. Sarah was known for having some psychic insight and was the other candidate fans pitched beside Diane being the person Gordon and Albert were referring to.
The second reason is the more obvious one; Diane has had a rough time getting on since Cooper’s disappearance. Whether there was an off-screen romantic aspect to their relationship—if so it would have been one-sided considering Cooper’s openness in talking about his conflicting affection for Audrey Horne and eventual relationship with Annie Blackburn—is unknown. Regardless of the nature of their relationship, it will be important factor to the FBI’s uncovering of the imposter Cooper and the journey of getting the real Cooper back and out of the overstretched Dougie subplot.