Fritz was the king of Eldia around 2,000 years ago. When one of his slaves, Ymir, gained the power of the Titans, he used her to expand Eldia into an empire and destroy his enemies. As a "reward," he made her his concubine and had three daughters with her. He and Ymir are the ancestors of every Subject of Ymir.
Upon Ymir's death, Fritz forced his daughters to eat their mother's body to preserve her power. Upon his own death, he commanded his daughters to continue reproducing and eating each other, so that Ymir's people will never die out and Eldia could rule the world with its Titans forever.
Fritz was a bearded older man. When Eldia was a small tribe, Fritz was shirtless and wore an animal pelt around his hip. As Ymir began amassing wealth for him, his attire gradually became more and more luxurious. He wore a horned helmet for a period, before transitioning to a crown decorated with five nine-pointed stars.
Appearance: Fritz was a bearded older man. When Eldia was a small tribe, Fritz was shirtless and wore an animal pelt around his hip.
Personality: Fritz was a man most strongly characterized by his limitless hunger for power and control. His ultimate goal had been world domination, and he was willing to accomplish that goal post-death.
History: Fritz was the ruler of the Eldians, who were originally a small tribe. He presumably ordered the attack and enslavement of the village that Ymir originally inhabited.
Trivia: In the original publication of Chapter 139 on , Fritz was incorrectly called "Karl Fritz", much like one of his successors. This was fixed with the release of Volume 34.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.