In the MarbleHornets series, Slender Man's (in this series known as The Operator) behavior is mainly that of a stalker, often chasing or stalking members of the cast, and, except for Alex, tends not to get directly involved with the cast, often chasing them down and hunting them for extended periods. Although vague for a while, Entry #55 suggests that Tim was being stalked by the Operator for years, which sadly roped in more victims; Entry #66 supports this.
(Note: In this article, Slender Man is referred to as the Operator, as in MarbleHornets)
Entry #1 starts with Jay noting he has noticed something unusual in the tapes. He decided to upload the data online for other opinions. The tape isn't footage of Marble Hornets, but rather is footage of Alex at home. Jay notes the audio in the tape has been cut, so he isn't sure if anything is said during the tape, and isn't entirely positive why Alex would have cut the audio in the first place. Regardless, Alex walks over to the window after killing the lights, and slowly looks out the window. He spots a tall figure standing outside, and dives back behind the curtain quickly to hide. The tape ends there, and Jay notes he may have missed other instances of this. He will be looking back through to see if he finds anything.
Entry #2 is the first time Alex may have found out about the Operator. In this video, Alex is walking his dog Rocky and passes a street light. Under the street light in the road is a "really tall man". Rocky gets scared and pulls Alex home, and Alex desires to go back with a video camera later, recalling the events in narration as he drives. When he gets to the location, however, the "tall man" is gone. Alex does note that he was near the site for Marble Hornets.
Entry #4, Entry #6, and Entry #10, contain various scenes of Alex trying to avoid the Operator, with several entries between showing static or noise problems. Also in Entry #7 the Operator appears in the background after they are finished with the scene for Marble Hornets.
Entry #11 implies that Slender Man has full access to Alex's house, as Alex lays down, and a shadow creeps across the wall and papers are moved as if by the "wind". Since Masky and Hoody hadn't been introduced yet, it can only be surmised that this is the Operator creeping around his house, meaning he stays outside the house just to taunt Alex. Also what appears to be a silhouette of the Operator can been seen around 1:04 thus further showing/ indicating that he is just taunting/ tormenting/ toying with Alex.
Entry #12 is yet another occurrence of Alex not knowing who the Operator is, and the crew tries to get him to leave the field they're shooting in, but the end result is uncertain. This shot probably takes place before Entry #2 due to the shocking appearance of the "tall man" not being mentioned, and everyone treats the Operator as just a passing civilian in the shot.
Entry #13 contains Alex once again encountering the Operator in the park that he is using to film Marble Hornets.
Entry #14 contains video involving Jat getting worried over the footage of Alex, who goes to sleep, only for the Operator to walk into the room. Alex gets up but never seems to see the Operator in the room, assuming he just "vanished" or Alex is not completely conscious (as is typical of proxy behavior). When Alex gets up, his face is covered in blood, and he blankly stares at the camera.
The Operator doesn't show up again until Entry #17, where he is standing outside the window, but nobody (except the unmanned camera) sees him, as they rehearse a scene for the movie. Jay does not recall the day despite being there and conscious the whole time.
Entries #18 and #21
The Operator's presence is implied in Entry #18, and Entry #21.
Entry #22 has Alex and Seth wandering a dilapidated building. Although they never see The Operator, Seth is grabbed and yanked away by an unknown force, to which Alex wakes up back in someone else's house and states he can't remember the evening minus what's on the tape. A shadow looms over Alex as well at this point, indicating the Operator is in the house.
Entry #23 is the first time Jay meets the Operator one on one. He wanders an Maintenance Building after magically being transported there while chasing Masky, however he hears a quiet voice calling out to him in the dark. Eventually he runs smack into the Operator, when he promptly wakes up the next day in his own apartment with the camera destroyed and the tape all that's left.
If the theories about the Operator's powers are correct, then the fire in Entry #25 is his doing.
In Entry #26, Jay gets a package and inside is a tape. In the tape, Alex and Amy are inside a house and when Amy begins fiddling with Alex's old camera, the Operator shows up in the house instantly and chases them, forcing Alex and Amy out the upper floor window.
Entry #27 has Jay awaken to static, noise, and massive electronic misbehavior, indicating the Operator was nearby, however he is not seen.
In Entry #28, and multiple entries forward, Jessica, whenever on camera, causes the same static that other people stalked by the Operator exhibit, indicating that she is somehow tied to the Operator.
Entry #29 has a person assumed-to-be-Jay wandering a pipe in Rosswood Park and finding blood, a bloody rock, a bloody T-shirt, and the Operator. The video cuts out and Jay can't recall if it was him.
Entry #36 has what may be the Operator shown on the right side of the road in the corrupted footage, with 2 black lines (assumingly his legs) leading up to a white pixel where his face would likely be.
Entries #40 and #42
Entry #40 has Jay enter the woods, fed up with waiting for Alex and running into the Operator. He tries to get closer to Jay, and Jay drops his camera and runs. In Entry #42, when talking to Alex, Jay admits he ran into it in the woods, and that he left his camera.
Entry #43 has Alex wander into a small grotto in the woods, and sees the Operator. He drops to the ground and coughs, then gets up and slowly walks towards him for no explainable reason just before the camera cuts out.
Entry #44, aside from Alex behaving oddly, has the Operator show up in his room and just stand there for a moment, before both him and Alex vanish.
Entries #46 and #47
Entry #46 has Jay break into Alex's apartment, and the two get ambushed by the Operator in the hallway. When Jay takes off, he notes Alex does not run, and in Entry #47, when he asks Alex why he didn't run, Alex doesn't give him an answer, instead yelling at him for not helping.
For Entry #48 we find that Jay has followed Alex into the woods, After having lost site of Alex he continues on in hopes to relocate him and continue on the trail to the truth. Jay gets scared and looks behind himself only to see nothing, however as the camera quickly pans back towards the left and forward the the Operator can be seen in the background around 4:35 without Jay noticing him.
Entry #49 involves Alex killing a man. After the man is dead, the Operator shows up for a moment and the body vanishes. Where it went or why he took it is uncertain, and Alex doesn't seem remotely bothered by these events.
Entry #51 once again takes place during the filming of Marble Hornets, and involves both Alex and Brian shooting on location in a burned out building. Brian becomes very nervous and scared, however before anything can be done, the Operator shows up. Brian runs through the building after Alex vanishes looking for him, and finds Tim, but when he turns around, the Operator is in the doorway and the camera cuts out. When the camera goes back, Brian's body is being dragged into a room, and the camera is picked up and walks out of the building, assumed to be Alex.
After the encounter with Alex in Entry #52, both Jay and Jessica are ambushed in the hotel. Jay hears at first Jessica screaming and tries to get to her, but is unable, then moments later, the Operator pulls Jay towards him and proceeds to attack him and cause weird oddities on the camera, before slamming him into the ground and leaving him there. Jay then wakes up in Entry #27.
In Entry #54, during the power outage, Tim is looking for the breaker box. During the out, the power returns briefly, and Tim scans the camera across the room, spotting the Operator standing in the corner. Tim doesn't make note of him, and the power leaves again shortly afterwards. Although the Operator makes no more physical appearances in this episode, he does emit static still, indicating he is following the group in the dark. The end of the video indicates that just before the camera ends, the Operator wanders into the room in full view of the crew. Since the tape ends, whether they reacted or not, or if they even remember the encounter are left unanswered.
The Operator makes no physical appearance here, however it is to note that during this entry, Tim notes that he has been feeling paranoid and nervous for years, often not getting sleep. This indicates that Entry #54 may have been one of many times the Operator bothered Tim, and has been stalking him for years.
During the video, Alex suddenly assaults Tim who has taken the time to relieve himself. After the blow, the camera seems to flip, revealing the Operator to the camera. The camera then momentarily flinches and when the image returns, the Operator is gone.
In Entry 57, the Operator makes no appearance, however his presence can be assumed. Towards the end of the video, Tim tries his best to stifle his coughing, but is visibly failing. Eventually the coughing attracts someone to him. It can be assumed to be either the Operator himself, or Alex who's electrical interference has grown incredibly.
After Jay and Tim see Hoody at the abandoned hospital in Entry# 58, Jay returns alone to explore the maintenance tunnel he saw Hoody run into. He eventually comes across a folder containing medical documents pertaining to Tim, as well as the white doll seen in previous entries. As he is leaving, his camera begins to pick up interference and he starts running after seeing something behind him. When he turns around again, the Operator is there. Jay escapes, but states at the end of the entry that he doesn't remember anything after picking up the documents.
Jay and Tim explores the various sites in Rosswood Park, revisiting the shack in Entry #52 and the building where Alex threatened to shoot Jay. Upon arriving to the Tunnel where Alex killed the pedestrian, Alex leaves a message on Jay's phone saying "LEAVE NOW." The Operator appears soon after and Tim collapses, as Jay tries to pull Tim back up, the Operator inches closer and closer, eventually forcing Jay to flee.
From Tim's point of view, caught on footage from the camera on his chest. As Tim has a coughing fit in the tunnel, the Operator is once again visible right behind him.
The Operator does not physically appear in this Entry. However, it is in this entry where Tim reveals that he believes he has been stalked by the Operator for years, and that he possibly got everyone else involved with the Operator as well.
As Hoody is beating up Alex in the Abandoned Hospital, and is about to shoot him, the Operator appears, and forces Hoody to flee.
As Alex is wandering a playground (the same playground from Entry #4) the Operator appears, and Alex charges him, yelling "What Do You Want!" When Alex runs into him, the Operator causes such massive distortion that it is impossible to tell what happened. After that, he vanishes.
The Operator isn't seen, but the static on the camera and the way Alex attacked Jay so suddenly implies he was around near the house.
The Operator is seen a few times in the video. Jay looks outside the window of the house, and alarmingly ducks out of cover. The Operator is not seen at that point, but the way the camera went slightly static implies he was there. He is seen again after Jay puts the camera on the window sill to help Tim open a door. After the two go down to the basement, with the outside now being pitch dark, with the sudden realization it is night by Jay and Tim. The two encounter the Operator in the basement before leaving the house. He is last seen after Tim jumps the fence outside, after Jay disappears behind him. The Operator appears after Tim finds Jay unconscious on the ground. Tim drops the camera and seemingly walks towards the Operator, soon becoming dazed and falling to the ground. He then disappears after the camera has a static attack and the two get back in their car.
The Operator isn't seen, but the static that appears throughout the video and the way Hoody began to collapse and cough indicates he was there.
The Operator isn't seen, however he probably was stalking Jay and Tim, as indicated by the way Jay suddenly attacked Tim and the static.
The Operator is first seen in the woods, then Jessica runs over where he was but is then gone. His 2nd appearance is when he attacks Jessica. His 3rd appearance was during the video shown after Jessica was attacked several times.
The Operator appears in the room Jay flees to after his encounter with Alex, where he tilts his head and reaches out to take Jay. He disappears afterwards when Alex comes in.
So far in MarbleHornets he is doing the following.
- It is believed Alex is working through him.
- It may be possible Jessica is doing the same now.
- Masky and Hoody are either working for him or against him, it's hard to tell.
- Jay is constantly stalked by him, but was not ever killed or taken over like the others until Entry #80.
- Slender Man interferes with electrical equipment, and it is shown this extends past cameras.
- Slender Man can "Slender Walk"
- Slender Man tends NOT to attack his targets (minus Entry #52) and is far more passive aggressive.
- Slender Man causes anyone nearby to become very sick and collapse.
- Slender Man specifically wanted Alex as is indicated by the fact that although most of the cast saw him in Entry #12, however Entry #55 shows that Tim has had the most interaction with The Operator (even if he is unaware of it).
- Why he is taking Alex is uncertain, and why he wants Jay is also uncertain.
- Jay may be due to getting involved with Alex, and Alex may be due to involvement with Tim.
- What Tim did to get stalked for years is uncertain.
- Even after years of alienation, Slender Man can find you instantly.
- Proxies in Marble Hornets do not appear to be aware of their personality shifts. Whether this is The Operator's fault or not is unknown.