Karl Maxwell is Noah Maxwell's grandfather that he visits on his trip to Milo's funeral video in New York. Karl has The Journal, which The Observer has noted as being "untouchable" as it prevents Slender Man or The Observer from directly interfering with him.
According to the video "My Grandfather Karl", Karl was born in Germany and lived near the town of Vöhrenbach, near the Black Forest. In 1944, he was a member of the Office of Strategic Services of the U.S. Army. Karl was sent on an operation near where he grew up. He and his group were disguised as civilians and sent to meet an informant who would give Nazi blueprints to the Allies. On the way there, they encountered an injured Nazi soldier that was running from something. They encountered a creature that Karl refers to as "Der Großmann". It looked like a tall man with no face and tentacle arms, wearing a black suit. He tried to talk to it in German, French, and Russian, but received no response. Karl and his group shot at the creature, but it didn't get damaged. It grabbed the Nazi soldier and started pulling him into the air. The creature and the soldier disappeared without a trace. He then told Noah that he was in grave danger and needed to avoid Slender Man as much as possible before kicking Noah out.
According to The Observer, Karl is in possession of a journal that was written by a man named Sebastian (who is now deceased) during the course of a big event that was not documented by historians, and contains forbidden knowledge relating to The Slender Man and The Collective. It also relates to an event that the Observer said that should not be known about.
Later, in the "Northern trip footage" video, Noah once again arrives at Karl's house. Karl does not answer the door when Noah knocks, but does respond by telling Noah to go away in German and that it was not the right time. Karl also appears in a couple of the Observer's videos, INTERRUPTION and HAPPYBIRTHDAY.
On Noah's ask.fm account, when he was asked a weird thing that happened to him as a child, he said that he had been in a boat with Karl, his grandma, and his mom, and the boat capsized. His grandmother drowned, and Noah stated that Karl looked for a book instead of trying to save his grandma. After this, according to Noah, Karl became a recluse and his parents stopped talking to him. Noah suspects the book to be The Journal.
- Many fans speculate him to be the proxy Persolus.
- It is also believed that the Nazi that he watched the Slender Man take away was Sebastian. This is backed up by the fact that the Observer once said that Karl and Sebastian didn't know each other, but they once met.