Karl Fritz

The First King was a Titan Shifter who created the Walls, and modified the memories of the humans inside.

Story
Long before the events of the story, the First King created the three Walls that would later be known as Wall Maria, Wall Rose, and Wall Sina. After the humans were inside, he used the power of the Coordinate to alter their memories, erasing everything about the world outside the Walls.

This power would later be passed down to the members of the Reiss family, by having the successor transform into a Titan and devour the predecessor, thus gaining their Titan Shifting abilities. Each member who took the power would inherit the lost memories of mankind, as well as the ideology of the King, keeping them from sharing these secrets with the rest of humanity.

Trivia

 * Although it is never mentioned, we can assume that the First King is a member of the Reiss family.