Marble Hornets is the largest and most famous series in the Slenderverse. It stared on June 20th, 2009.
The series tells the story of a young film student called Jay. After his friend, Alex Kralie, abandoned a film project for college called Marble Hornets for reasons unknown, Jay is given his tapes and, as he starts to watch them, notices that strange things had been happening to his friend.
Jay starts to get paranoid like his friend Alex was and there begins his hunt for answers.
This section may contain spoilers. Read at your own risk.
Jay is the main character of Marble Hornets. He begins the series by posting an introduction video, in which he informs the viewer that he had recently found a bunch of old tapes from his friend Alex Kralie's abandoned film, Marble Hornets. He notes that during the filming of Marble Hornets, Alex began to act oddly, and that the tapes have a lot of odd moments on them. As he posts clips from the tapes to YouTube, he starts to realize that the story they tell is a much darker story than simply personality changes brought on by stress, and he begins to succumb to the effects himself, going to great lengths to understand what is going on around him.
Alex is the original film student who wrote and directed the film "Marble Hornets." During the course of filming, he began to suffer from stress, paranoia, and other adverse effects as a result of being stalked by the Operator. After abandoning the project, he intended to burn all the tapes, but was persuaded by Jay to hand them over to him. At this point, he went off the radar for some time until his girlfriend found an old camera, and by filming, apparently summoned the Operator back into their lives. Alex contacted Jay after this, asking him for help. During the course of their working together, Alex grew increasingly untrustworthy, to the point of cornering Jay and Jessica in an abandoned building and threatening them with a gun. His current whereabouts are unknown.
Tim was, at first, one of the actors from Alex's movie project, Marble Hornets. In the movie, he played Brian's good friend, which he actually was in real life. By extension, he was good friends with Sarah too.
He's first mentioned in Entry 5 and, throughout the series, Tim shows numerous signs of having serious health problems, such as frequent coughing and headaches (signs known in the Slenderverse as being due to "exposure" to Slenderman).
Jay first contacted him to talk about trying to finish Alex Kralie's movie, but then Tim got much more involved into Jay's problems than expected and it was revealed that Tim had problems involving the Operator as well. He seemed, at first, not much relevant to the plot, but is now a very important character on the series.
The Masked Man, dubbed "Masky" by the fandom, is a recurring character who's always seen wearing a mask. It's not clear whether Masky wants to help or hurt Jay, or even if he's just trying to use him.
It's speculated that the Masked Man is, in fact, totheark because there were a few sights of him on a few videos in totheark's channel.
Later in the series, it's made clear that the Masked Man is Tim.
It's theorized that Masked Man and Hooded Man collaborated sometime.
He's an unidentified person who wears a beige hoodie and a black ski-mask. His intentions, motives and origins are still object of speculation, but no information has been confirmed to this day. He's called "Hoodie" by the fandom. By the end of the series, it appears that it was Brian.
The Operator is how the Slender Man is called in the Marble Hornets universe. He's a strange and tall figure who has no face and wears a suit. He's connected to the Operator Symbol (often taken as the main symbol that represents the Slenderverse).
Jessica is a character introduced first while Jay went through the footage of his missing six months. She was first introduced as the only non-staff person Jay had noticed on the videos of his stay in the hotel near Rosswood Park. She frequently had strange noises going on in her room which seemed to rouse Jay's curiousity about her. Sometimes she was friendly to Jay and sometimes put off by him. Toward the end of the footage from the missing time period she confronts Jay about his inconsistent lies for reasons of being at the hotel and reveals that, like Jay at the time, she couldn't remember why she was there either. She disappears shortly before the pair can run from the hotel, presumably displaced by Masky. Her current whereabouts are unknown though Jay later comes to learn that she was a friend and roommate with Alex's now equally missing girlfriend Amy, and that she and he willingly went to the hotel together while trying to find Amy, before losing their memories.
Brian seems to be the original link between Tim and Alex and was acting as the main character in Alex's college film project, Marble Hornets. Tim relates in Entry 16 that the only reason he joined the project was because Brian introduced him to Alex. It is Brian's house that Jay investigates in his search for his missing friends because of a hunch that Alex and Brian might be there. Unfortunately it is not Brian he meets there. Brian, like Sarah, has not been seen since the shooting of Marble Hornets, though like her he is the subject of several theories about the identity of totheark or the Hooded Man (which may be one and the same). He was included in Alex's recorded monologue among the list of people who are "gone." He is the one person that Alex does not show any aggression toward at any point during season one.
It later appears that he was the Hooded Man (and totheark, although sometimes Tim also uses this term for himself, for example, on Instagram).
Sarah was an actress for Alex Kralie's movie project, Marble Hornets. She's friends with Tim and is Brian's ex-girlfriend. Not much is said about her throughout the series and she hasn't appeared in many entries.
Some believe that she might be the Hooded Man.
Seth was the cameraman for the original Marble Hornets film project that Alex began in college. Not much is known of him past aside from that he has a dog and apparently a lot of patience compared to the rest of the crew of Alex's film project. Despite Alex's frequent frustration toward him, Seth seems to have remained a friend or companion of Alex's. The last thing we see of Seth chronologically is him being dragged away from Alex in a dark basement. Their reasons for being there are still unknown as well as Seth's eventual fate. However, Seth is later recounted as "gone" in the same list as Tim and Jay so it's possible that he is still out there somewhere and simply unaware of elements of his past.
Not much was said about Amy, except she was Alex Kralie's girlfriend and that both of them went to separate colleges. She was a friend and roommate of Jessica. Her first on-camera appearance was on Entry 26.
totheark is a YouTube channel (and presumably the name of the person who runs the channel) which replies to several of Marble Hornets' entries. The videos often contain missing footage from Jay's or Alex's tapes, with not only heavy audio and visual distortion but also coded messages and unexplainable symbols.
It's theorized that totheark took over Marble Hornets' channel more than once, posting its own entries. Sometimes Jay leaves them or re-uploads them.
totheark's identity and motives remain unknown.
Apart from the YouTube channel, Jay runs a Twitter account in which he posts links to the videos that are uploaded to YouTube and things that happen that are not on tape. Coded messages are often tweeted by someone yet unknown.
The Slender Doll first seen in Entry #18 is an iconic symbol of Marble Hornets, often being referenced in other series or blogs. It is somehow connected to Slender Man, but whether it is used to keep him away or to "call" him, it's still unknown.
This section may contain spoilers. Read at your own risk.
Rosswood Park was first mentioned in Entry #31 and first seen in the footage from the seven months Jay lost his memory. It is an important location in which several entries have been filmed.
The Hospital is located inside the Rosswood Park and near the abandoned building. It was used on the original Marble Hornets shoot as Brian's old school. According to some entries, it has been theorized that Tim has a personal connection to this location.
Also known as "the annex", the abandoned building is located near the hospital inside Rosswood Park. Though Tim was more cautious about it than he was about the hospital, he still agreed to lead Jay there.
The Tunnel is another key location in Marble Hornets. Both Alex and Jay have experienced and taped major events in this place.
The Red Tower first appeared in Entry #5 and is the place where Jay found tapes containing footage from Entry #22. Near the Red Tower, Jay also found numrous burned tapes (presumably the ones Alex previously tried to destroy).
All the videos shot near the Red Tower show heavy audio and video distortion. There's also a very visible Operator symbol on the side of the tower.
Not much is known about this other key location of Marble Hornets. It's only theorized that it was Brian's house or Alex's house from three years prior.