Ymir was a graduate of the 104th Training Corps and a former member of the Survey Corps. She also had the ability to transform into the Jaw Titan, and had knowledge about the true nature of Titans and the history of the world outside the Walls.
Ymir was a tall, slender young woman with short, shaggy black hair parted down the middle. She had deeply intimidating gray eyes and freckles on her cheeks. Ymir's hair was usually kept in a low ponytail with a white tie.
Initially, Ymir appeared to be selfish, cynical, uncooperative, and confrontational. However, she later revealed a kinder side, especially around Historia Reiss. Even earlier, Ymir showed this emotional and selfless aspect of herself when she took full responsibility for the Eldian cult's existence, despite her followers betraying her first. She also saved Erwin Smith's life, even though she blamed him for the failure of her plan of ensuring Krista's future. It should also be noted that she showed considerable regret over devouring Marcel Galliard who was Reiner Braun, Annie Leonhart, and Bertolt Hoover's friend and comrade, regardless of the fact she did not do it consciously.
Appearance: Ymir was a tall, slender young woman with short, shaggy black hair parted down the middle. She had deeply intimidating gray eyes and freckles on her cheeks.
Personality: Initially, Ymir appeared to be selfish, cynical, uncooperative, and confrontational. However, she later revealed a kinder side, especially around Historia Reiss.
History: Many years ago, a nameless Eldian girl lived in the streets of Marley as a beggar with no home. Eventually, she was found by a man who gave her the name "Ymir," after the ancestor Ymir Fritz, and brought her into a group dedicated to the worship of Ymir.
Ymir in the story: " Ilse's notes of the encounter were later discovered by Hange Zoë. These were the first hints of Ymir's true nature. On the first day of training, Ymir and Krista Lenz venture outside and fetch refreshments for Sasha Blouse after her 5 hour-long run as a punishment.
Source: Attack on Titan Wiki, available under CC BY-SA 3.0.