Karl Fritz was the 145th king of Eldia and was the one responsible for its downfall. After the Eldian Empire's collapse, he moved many of the Eldians to Paradis Island where he raised the Walls, and used the power of the Founding Titan to erase the memories of the outside world from most people inside. Afterward, he changed his family name from Fritz to Reiss and became the first King of the Walls.
Karl Fritz's Titan form stood at 13 meters tall, like all other Founding Titans after Ymir Fritz. It had long, straight hair and 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.
Karl Fritz was a man wracked with the guilt of his ancestors. Even before inheriting the Founding Titan, Karl Fritz detested the atrocities committed by the Eldian Empire. This intense guilt would lead him to conspire with the Tybur family to weaken Eldia enough for Marley, a nation long oppressed by Eldia, to conquer it and take back their land.
Appearance: Karl Fritz was an aged man with long fair hair and a thick beard. Karl Fritz's Titan form stood at 13 meters tall, like all other Founding Titans after Ymir Fritz.
Personality: Karl Fritz was a man wracked with the guilt of his ancestors. Even before inheriting the Founding Titan, Karl Fritz detested the atrocities committed by the Eldian Empire.
History: Over a century before the Battle of Trost District, Karl Fritz was crowned the 145th King of the Fritz family and inherited the power of the Founding Titan. For many hundreds of years, various families subservient to the Fritz family were constantly at war with each other for control over the eight Titans, a civil war which Karl Fritz was ashamed of.
How Karl Fritz echoes later: 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 and Frieda Reiss's view of the walled people as sinners. Beyond the Walls, the actions of Karl Fritz inspired rebellion in Marley.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.