Slenderman is a character who first appeared in Super Mario 64 Bloopers: My Best Friend Slenderman. He also appears in the opening video that automatically starts playing of oneself when you visit Supermarioglitchy4's channel. Unlike his traditional depictions as a horror icon and villain, Slendy is here portrayed as friendly, even having befriended Mario.
Slenderman is a Creepypasta Figure and appears to be friendly with Mario and SMG4. He stands 1 1/2 times Mario's height. Slenderman also shows his concern for Mario, as seen in SM64 Halloween 2016 - SPAGHETTI.EXE, saving him numerous times from the various Creepypastas. He has also made a Cameo appearance in multiple previous episodes.
Super Mario 64 Bloopers: My Best Friend Slenderman
On a dark night, Toad wants Mario to go to the shop. When coming to the gray, dark scission Mario passed in previous bloopers, Mario could either go via Gay Castle or a forest. Mario chose the forest. On a glade, Mario hears sinister sounds and a ghastly voice. He then spots Slenderman, unaware of who he is, and takes him to the castle. Everyone but Starman3 runs away from Mario. Starman tells Mario who Slenderman is but Mario doesn't believe him. When searching for a job for Slenderman in the borough, Starman3 and some constables appear. Starman3 declares Slenderman (or "Slendy", which is how Mario calls him) under arrest. Though, Slenderman uses the Slender-Copter, but crashes with Mario on Wario's parents as Starman3 watches from the high building Mario and Slender Man are. Later, in the castle, Mario flushes Slenderman trough the lavatory after the latter starved.
M64 Halloween 2016 - SPAGHETTI.EXE
The Episode is formatted in a game-esk mode, where the Game asks the Player, Frankie, if he dares to play. When the Game begins, Mario buys a Cursed Spaghetti Can from the Happy Mask Salesman. Mario then consumes the Can of Spaghetti, which causes him to perceive multiple Creepypastas such as Ben, Sonic.exe, Tails Doll, Jigsaw, Herobrine, Chucky, and Jeff the Killer respectively. Slenderman shows his soft side for Mario as he defends him against the Creepypastas. Later in the Episode, Mario gets an Exorcism done, and the Ben from earlier starts to copy himself all around inside the Building. Slenderman comes to the rescue again, saving Mario from the multiple Bens. However, when he escapes, Slenderman finds out that Mario has been turned into a Ben, initiating a Game Over.