Marble Hornets is one of the biggest influences on the Slender Man Mythos.

SHIPS:
Jay Merrick x Jessica Locke - Just cause.
Brian Thomas x Sarah Reid - During Marble Hornets' production.
Seth Wilson x Sarah Reid - After Marble Hornets' production.
Alex Kralie x Amy Walters - Before he went insane (It's canon).
Tim Wright x Kate Hayes -  They both killed their best friends, they've had their minds taken over and they've been stalked for as long as they can remember (I know that MH is separate from the Slender series, but I can dream, can't I?)

HEADCANONS:
Jay Merrick - Jay was autistic and his search for Alex/to find out what happened, turned into a special interest, which is why he couldn't just stop the series. Jay was socially awkward and was shy around girls. Had Jay survived at the end he would have dated Jessica.

Alex Kralie - Alex was  friends with Brian since his childhood, probably being one of the children at Alex's birthday in enttry #37. Alex was the type of guy to write songs about Amy in his free time and never told anybody.

Brian Thomas/Hoodie - Brian made the Operator Doll while suffering from the Sickness. Brian also dated Sarah briefly during the filming of Marble Hornets. Alex noticed this and added it to the Marble Hornets script for more drama.

Tim Wright/Masky - During his first few days of college, Tim took too much of his medication and started choking. Luckily, he was saved by Brian, who became his first real friend. After the series ends he manages to restart his life and gets a girlfriend named Kate Hayes/The Chaser, who is also being stalked by the Operator.

totheark - I believe that totheark is either Seth or Sarah.

The Ark - My interpretation of the Ark would be a twisted version of Rosswood Park. In the sky there would be a solar eclipse that gives off a faint light. The grass would be dark gray and the trees would have no leaves. On the branches of some of the tree would be the stretched, decaying corpses of the criminals that Alex spoke of in his story. The landmark structures would look burnt on the outside, but on the inside the walls and the floor are covered in Operator pages and blood stains. The YouTuber Night Mind believes that the Ark is where TO takes the corpses of Alex's victims after he kills them. And according to Always Watching, TO has the ability to reanimate corpses. So if a living victim found their way to the Ark, TO would send a bunch of these corpses (or as I like to call them "Arkers") to kill them.