Netflix's The Watcher ends on an ambiguous note, but the series makes a good case for John Graff being the titular letter writer. Partly based on a true story, The Watcher follows the Brannock family as they move into their dream home in Westfield, New Jersey, only to begin receiving typewritten letters containing personal information and threats against them. There are many plausible suspects presented in the series, but the most likely identity of the Watcher is the home's previous owner, John Graff.
Graff murdered his entire family in the home in 1995, then apparently fled the town, but not before cutting his face out of every family photo. The murders were only corroborated by one police file and were never reported on in the press. Despite the murders' status as an urban legend and The Watcher's ending remaining ambiguous, context clues point strongly to a character who goes by the name "William" as not only being the true John Graff but also the titular Watcher.
Why John Graff Came Back to 657 Boulevard
Of all the potential suspects presented in The Watcher, in the end, John Graff is the only one with the insidious combination of means, motive, and unencumbered opportunity. The series establishes that 657 Boulevard has an ethereal power over people. Tormented by his dysfunctional home life and an emasculating public persona, Graff slaughtered his family, and it's possible he never actually left 657 Boulevard because he couldn't bear to do so.
Graff could have continued living in the secret room underneath the house, forged a partnership with Pearl, and made her complicit in the letter writing — a woman had licked the envelopes, and a woman in pigtails similar to Pearl's was caught on camera in Dean's bedroom. Eventually, Graff would have been able to get a job at the public library under the pseudonym William. As he had cut his face from every photo, only longtime residents could have conceivably recognized him.
Why John Graff Was Probably The Watcher
John Graff is noted for his unsettling disposition, but among The Watcher's eclectic cast of characters, his creepiness is not an anomaly, hardly making him the most likely suspect. There also remains another issue against the idea of Graff being the Watcher: he too received letters. In Theodora's retelling of the story, Graff was tortured over these letters, dreading even opening the mailbox on the chance he might find another correspondence. However, this version of events was purely speculative since Graff never told anyone about the letters, instead leaving them to be found along with his family.
Graff's Watcher letters were also slightly different from the Brannocks'. While the Brannock letters focus on the house and the family's movement within the home, as depicted in the Netflix true crime series, the Watcher in Graff's case somehow watched the home during the day, followed his daughter when she would sneak out, and knew that he was fired from his job in the city. Even for the Watcher, this level of insight would be impossible unless Graff himself was writing the letters, either to establish a motive for killing his family or as a symptom of his fracturing psyche. With that being the logical case, combined with other clues, The Watcher points to John Graff as probably being the true titular terror.