Ymir lived as a homeless Eldian beggar in Marley before a man took her in, naming her after the legendary founder Ymir Fritz. She became the figurehead of an Eldian cult that worshipped her as a goddess, bringing joy despite knowing she wasn't royalty. When authorities uncovered the cult, the man who named her betrayed her to save himself, claiming she deceived him. Ymir accepted responsibility and was arrested alongside her followers. Following public humiliation, including citizens stoning her, she and other cult members were forcibly turned into Pure Titans and exiled to Paradis Island. She wandered outside the walls for approximately sixty years in a nightmarish state with minimal awareness. In 845, while wandering as a Pure Titan, she encountered the Warriors—Reiner Braun, Bertholdt Hoover, Annie Leonhart, and Marcel Galliard—on their infiltration mission near Wall Maria. She attacked, attempting to devour Reiner, but Marcel sacrificed himself, pushing Reiner to safety and being consumed instead. This transferred Marcel's Jaw Titan power to Ymir, restoring her human form and consciousness in a desolate landscape she called "freedom." She traveled to the walls and survived as a thief within Wall Sheena for two years. There, she overheard Church of the Walls officials discussing an illegitimate royal child named "Krista Lenz," sparking her interest. She enlisted in the 104th Cadet Corps in 847 to find Krista. During training, Ymir appeared selfish, cynical, and confrontational. She dismissed Sasha Blouse's exhaustion after punishment and later criticized her for hiding her native accent with overly formal speech, accusing her of inauthenticity. Krista Lenz (later revealed as Historia Reiss) defended Sasha, creating tension. Ymir's perceptiveness allowed her to recognize Krista's martyr complex, Reiner's dissociative tendencies, and others' hidden insecurities. During a lethal snowstorm exercise, when cadet Daz collapsed, Krista insisted on rescuing him. Ymir deduced Krista intended to die heroically. Revealing knowledge of Krista's royal background, Ymir challenged her to live authentically. To save all three, Ymir transformed into her Jaw Titan—a 5-meter form with sharp fangs, claws, and disproportionate features—to carry Daz to safety. She made Krista promise to embrace her true identity, Historia, in exchange for keeping her Titan ability secret. Ymir intentionally underperformed in training to ensure Historia ranked in the top 10, enabling her to join the Military Police. Ymir graduated outside the top ranks and followed Historia into the Scout Regiment. In the Battle of Trost, Ymir was stationed with Historia, Connie Springer, and others. Finding Armin Arlert catatonic after his squad's annihilation, Ymir callously remarked that his survival wasn't worth his comrades' deaths, angering Connie. Historia mediated, and Ymir jokingly proposed marriage to her. Ymir and Historia later assisted in supply operations during the Titan incursion. Her Jaw Titan form resulted from inheriting Marcel Galliard's power after consuming him as a Pure Titan. Her previous Pure Titan form was emaciated with visible ribs and non-sharp teeth. As a human, she was tall and slender with short, shaggy hair often tied in a low ponytail, gray eyes, and freckles. She wore standard military uniforms with a dark v-neck underneath. After regaining her humanity, she resolved to live only for herself as defiance against those who exploited her. Despite this, she displayed selflessness, saving Erwin Smith despite blaming him for her plans failing and expressing regret over Marcel's death. Her pride prevented her from revealing Titan-related knowledge to the Scouts, even when it could have aided humanity. Her bond with Historia remained central, motivating her later decisions to protect her.