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Ymir is a tall, slender young woman with parted hair often styled into a ponytail and intimidating eyes. She has straight, long, brown hair held back with a red tie and golden eyes, as well as a slight tan.
Initially, Ymir appears selfish, cynical, uncooperative, exploitative and confrontational, but later reveals kindness, especially around Christa Lenz. She often criticizes people for being untrue to themselves, as when she berated Sasha Braus, who developed an extremely polite and formal way of speaking to hide her native accent.
Ilse Langnar, a member of the Scout Regiment that resembled Ymir, encountered a Titan that spoke of her as a "subject of Ymir." This Titan mistook Ilse for Ymir, and bowed before her while referring to her as "Lady Ymir." Ilse's notes of the encounter were later discovered by Hange Zoë and are the first hints of Ymir's true nature.
Some time shortly after Wall Maria fell Ymir as a Titan ambushed Reiner Braun, Bertholdt Hoover and their friend as they were fleeing to Wall Rose. While Ymir almost grabbed Reiner his friend pushed him away, and Ymir grabbed him instead and devoured him.
On the first day of training, Ymir and Christa venture outside and fetch refreshments for Sasha after her 5 hour-long run as a punishment. Ymir acts apathetic and shows little care for Sasha's well-being, telling Christa that she could use Sasha's stupidity to her advantage.
Ymir graduates in the 104th class outside the top 10, which Christa deems that she deserved to be there more than her. It is implied that she intentionally gives less than her full effort so that Christa could have the 10th slot. After graduation, she follows Christa into the Scout Regiment.
Ymir is in the Trost District when the Colossal Titan breaks down the gate. She is part of the squad that discovers a catatonic Armin and mocks him upon learning that he is the only survivor. This results in a brief and heated exchange with Conny Springer before Christa steps in to mediate. In response, she requests that Christa marries her once the battle is over. She is later among the squads able to retreat over the Wall when the order was given and waited on standby for further orders.
Ymir possesses great physical ability - she shows to be fully capable in combat and use of weapons. During her time as a trainee, it has been rumored that she started slacking off to ensure Christa Lenz a spot in the top ten.
- Christa Lenz - She is romantically attracted to Christa, and utterly devoted to her. Her primary goal is to protect her no matter the cost. The two have a strong connection, and deeply trust each other even while keeping secrets.
- Her name Ymir is the very same as a primeval being in Norse mythology who is the progenitor of all Jotun, or ice giants, and the first living being. Ymir also means "the scream".
- The official website mentions Ymir is in love with Christa. Also at the series panel for Animagic 2014, producer George Wada confirmed that Ymir and Christa are a couple.
- Since Ymir did not given her true name in the first season of the anime, she was given the name "Freckles" (そばかす Sobakasu?) during the credits.