Ymir Fritz was the first person to obtain the power of the Titans. She was a slave of the Eldian king, who abused her power to bring prosperity to Eldia while devastating Marley and the rest of the world. Thirteen years after awakening her power, she died protecting the king; her corpse was then forcibly cannibalized by their daughters, Maria, Rose, and Sheena. Her powers then split into what eventually became known as the Nine Titans.
Upon her death, Ymir was transported to the Coordinate, where she molded Titans out of the sands and obeyed the royal family whenever they sought the Founder's power.
Ymir Fritz was small in stature with light hair. She had straight cut bangs with shoulder-length hair that later grew down to her shoulder blades with age. As a slave, she wore tattered clothing with a cloth headband, sash belt, and leather sandals laced above her ankles. After entering the favor of the Eldian King in adolescence and adulthood, Ymir wore a sleeveless dress with thin decorative bands around each arm. Like all slaves of the Eldian tribe, her tongue was cut out when she was captured, though it may have regenerated after acquiring the power of the Titans.
Appearance: Ymir Fritz was small in stature with light hair. She had straight cut bangs with shoulder-length hair that later grew down to her shoulder blades with age.
Personality: Being a subject of ancient history, there are various interpretations of who Ymir really was. Frieda Reiss described Ymir as a kind girl who was always thinking of others.
History: Around 2,000 years ago, Ymir's village was attacked and enslaved by the Eldians, who also murdered her parents and cut out the tongues of the survivors. One night, Ymir opened the gate to a pen, allowing for some pigs to escape.
How Ymir Fritz echoes later: Some years later, when the Survey Corps discovers Grisha Yeager's basement, they find three journals. Within Grisha's writings, he recounts his childhood when he was told the story of Ymir and her people by his father. Grisha writes of joining the Eldian Restorationists in the nation of Marley several years afterward, learning of the supposed great accomplishments achieved in Ymir's time.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.