| The subject of this article is anonymous.|
The name used is an alias, nickname or surname found in an official source, but the actual in-universe name of this person is unknown in this continuity.
|◄ Preceded by|| First Reiss King|
|Followed by ►|
|Unknown||Unknown, eventually Rod and Uri's father|
|◄ Preceded by||First Reiss King||Followed by ►|
|Unknown, eventually Ymir Fritz||Unknown, eventually Rod and Uri's father|
The First Reiss King was the first ruler of humanity inside the Walls and prior to that he ruled also as the 145th Fritz King of Eldia. He possessed the Founding Titan which he used to create the Walls and alter the memories of humanity inside.
The first king's Titan is depicted with long shaggy hair and exceptionally sharp teeth. In contrast to its thick legs, the upper part of its body was quite skeletal, with its ribs visible through the skin. Its arms were thin and long, similar to those of the Beast Titan.
Over a century before the struggle for Trost District, the First King inherited the power of the Founding Titan. Rather than following the method of his ancestors and using the Founding Titan to maintain order in the nation of Eldia, the King abandoned the conflict, resulting in the Great Titan War. He moved the capital of Eldia to Paradis Island to the east of the mainland of their nation. In the King's absence, the oppressed society of Marley, who took part in the Great Titan War in Eldia, gained the power of seven out of the Nine Titans and rose up against their oppressors.
On Paradis Island, the King used the Founding Titan to cause Colossal Titans to form three concentric Walls around their territory: Maria, Rose, and Sina. Afterward, the First Reiss King used the Founding Titan once more to erase the Subjects of Ymir's memory of the Great Titan War and all of history before the raising of the Walls. However, some bloodlines were excluded, such as the Ackermann family and the Oriental clan. To prevent the risk of these unaffected groups spreading information about the world beyond the Walls, King Reiss offered positions of nobility in Wall Sina to these minority families, and most accepted the offer. However, the Ackermann family and the Oriental clan rejected King Reiss's ideology and turned against him. For this reason, King Reiss ordered for these families to be hunted down and exterminated.
When the Walls were completed, King Reiss gave an ultimatum to the outside world: if any act of war was committed against the remnants of Eldia, the tens of millions of Titans within the Walls would be unleashed upon the world to flatten the entire earth. However, this ultimatum was merely a bluff to ensure lasting peace. In secret, the King told his surviving relatives on the mainland that he made a vow renouncing war with the Founding Titan, ensuring that he and his successors would never use its power for retaliation in the event that the Walls would fall under attack. If the day should come when Marley's power was great enough to destroy Eldia, his successors would accept it.
To the east of Orvud District, King Reiss created a massive subterranean cavern where his successors would inherit the power of the Founding Titan. Though the King had died, his memories and his vow renouncing war would live on in the Kings of the Walls who inherited the Founding Titan after him for the next hundred years.
Throughout the generations, the ideology of King Reiss was carried through the monarchs of the Reiss family, and none would tell of the history of the Eldian Empire before the raising of the Walls. The King's beliefs would often manifest outwardly in his successors, such as Uri Reiss's desire for an age of peace before destruction.
Beyond the Walls, the actions of King Reiss inspired rebellion in Marley. The Restorationists, led by Eren Kruger under the alias "the Owl," sought to take the Founding Titan from the King of the Walls to finally rise up against their oppressors. The Restorationists believed that King Reiss was cowardly for abandoning his people in the Great Titan War, and by taking the Founding Titan they would restore the Eldian Empire to its former glory. Eventually, the movement was discovered and exiled to Paradis Island, though Restorationist Grisha Jaeger survived to carry out the mission. Inspired by Eren Kruger, who gave him his Attack Titan, Grisha entered into the Walls to begin his search for the Founding Titan.
During the fall of Wall Maria in the year 845, Grisha Jaeger visits the Reiss Chapel where King Reiss's cavern was delved. Confronting Frieda Reiss, he begs for the King of the Walls to stop the attacking Titans for the sake of her people. The vow renouncing war would not allow her to retaliate, and Grisha's pleas are refused. Taking matters into his own hands, Grisha battles Frieda's Founding Titan with his Attack Titan and steals the Founding from her, before slaughtering the rest of the Reiss family except for Rod Reiss, who escapes.
Fleeing the scene, Grisha gives his son Eren Jaeger a Titan injection, passing the Founding Titan and the Attack Titan to him. With no royal blood in him, the Founding Titan becomes unusable and the King's vow renouncing war is not passed on to Eren.
When Eren Jaeger and Historia Reiss are captured and taken to Rod Reiss, he tells the two the history of the Reiss family and the passing of the First King's ideology through the line of monarchs in the past century, preventing the truth of the world from being known to the people of the Walls.
During the Battle of Shiganshina District, Zeke Jaeger laments King Reiss's decision to take the memories of the Eldians away from them after the Great Titan War, leading them to make the same mistakes of the past.
In the aftermath of the battle, the survivors of the Scout Regiment discover the basement of Grisha Jaeger. In the basement, they find three journals describing his life before coming to the Walls. Through his story, the people of the Walls learn of the Restorationists' attempt to take the Founding Titan from the King of the Walls to end his vow renouncing war. With this information, the military of Paradis Island decides to inform the people of the history that King Reiss took from them a hundred years ago.