Karl Fritz

The First King was the unnamed Titan Shifter who created the Walls and modified the memories of the humans inside, erasing all the history of the world before that event.

Appearance
The First King's human form is unknown, but in Titan form he seemed to be of a very tall class. He had long straight hair and sharp teeth. In contrast to his thick legs, the upper part of his body was quite skeletal, with this ribs visible through his skin. His arms were thin and long, similar to those of the Beast Titan.

History
More than 100 years ago, humans were at the brink of the extinction due to the predation by the Titans. The First King, who possessed the power of the Coordinate, had the possibility to use it to wipe out all Titans, but instead wished for a world ruled by them.

He limited himself to use his power to protect the few survivors of humanity, by controlling some Titans and make them harden and encage themselves into the resulting structures, creating the three Walls that would later be known as Wall Maria, Wall Rose, and Wall Sina.

After the humans were inside, he used his power again to alter their memories, erasing all knowledge about the world outside the Walls and the history before that event. The only people unaffected by this manipulation were the Ackerman family and the Asian clans.

The First King later gave the Coordinate to the Reiss family, presumably dying in the process.

Legacy
The Reiss family continued passing down the Coordinate between them generation to generation long after the First King's death. This was done by having the successor transform into a Titan and devour the previous user; thus gaining the Coordinate, the Titan Shifting abilities, the lost memories of mankind, and even the ideology of the First King; keeping them from sharing their knowledge and from freeing humanity from their encagement.

Trivia

 * Since Rod Reiss stated that the Coordinate can only be used to its full potential by members of the Reiss bloodline, it is possible that the First King was a member of that family too.
 * Hange Zoë referred to the First King as "the original Titan".