Ymir graduated from the 104th Cadet Corps possessing the ability to transform into the Jaw Titan. She held extensive knowledge of Titans and the world beyond the Walls. Tall with disheveled brown hair often tied in a low ponytail and distinctive freckles, her appearance featured sharp, intimidating eyes. Her 5-meter Jaw Titan form displayed rugged dark hair, a disproportionately large head, sharp fangs, claws, and long arms, lacking distinctly feminine features common among other intelligent Titans. Her pre-Jaw Pure Titan form appeared emaciated with prominent ribs and regular teeth. Born a nameless orphan in Marley, she lived as a beggar until age four. A man then named her after Ymir Fritz and brought her into an Eldian cult. She embraced this role to bring joy, despite knowing her divine lineage was fabricated. At eleven, Marleyan authorities arrested the cult. The man who named her accused her of deception to save himself, but she claimed leadership to protect her followers. This sacrifice proved futile; the entire group was paraded, stoned, and forcibly turned into Pure Titans near Paradis Island. Ymir spent approximately sixty mindless years wandering outside the Walls, later describing this period as an "endless nightmare". In 845, she encountered Warriors Reiner Braun, Bertolt Hoover, Annie Leonhart, and Marcel Galliard during their infiltration. As a Pure Titan, she attacked them and devoured Marcel, inadvertently inheriting the Jaw Titan. This act restored her human consciousness near a desert landscape under a strange sky, which she called "freedom." She infiltrated Wall Sheena, surviving as a thief for two years. During this time, she overheard Church of the Walls members discussing Historia Reiss (then known as Krista Lenz), an illegitimate royal child. Sympathizing with Historia's plight, Ymir enlisted in the 104th Cadet Corps in 847 to find her. Initially presenting as selfish, cynical, and confrontational, Ymir frequently criticized others for inauthenticity, such as confronting Sasha Braus about hiding her native accent. Her interactions with Historia revealed a selfless and protective core. During a mountain survival exercise, she deduced Historia's martyr complex when Historia insisted on rescuing an unconscious cadet, Daz, despite near-certain death. Ymir transformed into her Jaw Titan to carry Daz to safety, revealing her power to Historia only after securing a promise that Historia would live under her true name with pride. Later, Ymir intentionally underperformed in training to ensure Historia ranked in the top ten, enabling her to join the Military Police. Stationed in Trost during the Titan breach, she witnessed Armin Arlert's trauma and callously remarked on his squad's loss, though she later volunteered for supply duties with Historia. Her perceptiveness identified Reiner's dissociative identity disorder and Bertolt's feelings for Annie. At Utgard Castle during a Titan incursion, she discovered canned herring with foreign text, accidentally revealing her uncommon literacy within the Walls. This mistake exposed her outside knowledge to Reiner. When Titans besieged the castle, she transformed to defend the group, prioritizing Historia's safety. Facing Reiner and Bertolt's revelation as the Armored and Colossal Titans, Ymir negotiated Historia's protection in exchange for her cooperation. She acknowledged her debt to them for inadvertently freeing her from her Pure Titan state by her consumption of Marcel, expressing regret for his death despite its unconscious nature. She joined them willingly, believing this ensured Historia's survival. Her final act involved writing a letter urging Historia to live proudly for herself, mirroring Ymir's own life philosophy forged through betrayal and exploitation. She was taken to Marley, where the Jaw Titan power was transferred to Porco Galliard around 850, implying her death.