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Ymir is a tall, slender young woman with intimidating golden eyes and middle parted hair often styled into a ponytail with long bangs at the sides framing her face. She has disheveled-looking brown hair held back with a red tie and usually wears the uniform of the Scout Regiment.
Ymir's Titan form is considerably smaller than most Titans, standing at 5 m and is the shortest Titan shifter thus far. She has long arms, a large nose, rugged hair, small black eyes, small pointed ears, sharp fangs and claws allowing her to easily climb trees and tear into enemy flesh. Unlike other Titan shifters, the body is not well-proportioned because her head is too big. Unlike the Female Titan, her Titan form possesses no distinct, feminine features. Overall, it lacks the distinct similarity to her human form that the majority of other Titan shifters show. Prior to eating Bertholdt Hoover and Reiner Braun's friend, her Titan form was noticeably thinner, and her rib cage was more pronounced. She is also shown to be capable of coherent speech in her Titan form.
Initially, Ymir appears selfish, cynical, uncooperative, exploitative and confrontational, but later reveals kindness, especially around Historia. 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.
After Nanaba reports that Wall Rose has been breached, the scouts are ordered to go on horseback and evacuate local citizens near-by. Ymir is seen next to Christa Lenz on her horse, accompanied by Nanaba and Henning. Both the recruits are devoid of their omni-directional mobility gear and Ymir initially voices refusal to be a part of the team as being unarmed would make them extremely vulnerable to Titans.
After the Western and Southern teams take refuge inside Utgard Castle, Ymir comes under suspicion by Reiner when she is able to read the label of the tuna can in another language correctly and he comes to suspect that she may know more than what she is letting on. When the castle comes under attack by Titans capable of moving at night, the four Survey Corps members are killed and remaining rookies are left under danger of the tower collapsing. Fearing the end is near, she tells Christa to live a life she will be proud of. Asking her to remember a promise that they made earlier, Ymir jumps from the tower and cuts herself with the knife Conny gave her, triggering her Titan transformation into a Titan.
Ymir fights the Titans strategically, aiming for the nape of their necks and showing great focus. Her transformation shocks her peers. However, while being fast and vicious, her Titan form is no larger or stronger than most Titans, and she is soon caught. The Titans attack viciously and Ymir reaches for the Wall but consciously releases it as the bricks come loose and the heavily damaged tower shows signs of falling over. Christa reasons that with her power, Ymir could have easily escaped alone, but she is putting her life on the line to fight for the recruits survival. Angered, Christa yells at Ymir to stop behaving like a hero and to start fighting for herself. She orders Ymir to tear the wall down. Ymir then breaks the tower and as the tower collapses, Christa, Conny, Bertholdt and Reiner hold on to her hair as Ymir in her Titan form told them to grab her to be safe. The tower completely collapses, burying the Titans under the rubble.
When the Titans begin to emerge from the rubble, Ymir launches at them immediately and puts herself at risk once again. However, she is pushed to her limit and finally overpowered. The Titans begin to tear her body apart and devour her. Christa runs to the Titan horde, begging Ymir to stay alive so that Christa could reveal her real name. Ymir and the recruits were saved by the timely arrival of Hange Zoë's squad, who proceeded to slay all the remaining Titans in the vicinity. Later, Ymir is shown lying almost unconscious in her human form, her right arm and leg missing, with Hange and Christa by her side. Before Ymir passed out, Christa reveals to her that her true name is "Historia".
The scouts regroup atop Wall Rose and Ymir is brought up via a lift. Keiji explains that her insides are similar to that of scrambled eggs and claims that any normal person in her condition would be dead. Historia is then seen speaking to Hange on Ymir's behalf, claiming that she is undoubtedly an ally to humanity. While Hange is skeptical, she notes that it would obviously be for the best if they could all get along and work together, noting that Ymir's information is a treasure to humanity.
Shortly after, it is revealed that Reiner and Bertholdt are enemies of humanity, and the true identities of the Armored and Colossal Titans. The two transform and Reiner tries to kidnap Eren who also transforms as well and begins fighting him. Meanwhile, Bertholdt partially transforms into the Colossal Titan atop the wall and battles the scouts, fending them off with steam. During his transformation he is able to grab Ymir and one other soldier and devours them. When it later appears that Reiner may lose his battle with Eren, he scream to alert Bertholdt of his situation. The Colossal Titan, partially dissolved from steam, then falls of the wall and heads straight for Eren and Reiner below.
Ymir possesses great physical ability in her human form - 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 Historia a spot in the top ten.
In her Titan form, she was shown to be extremely agile and quick, being able to maneuver effectively within a Titan horde, slaying several Titans in the process. She was also able to produce coherent speech from her Titan form, a feat that only two other Titans have been shown to be capable of.
- Historia - She is romantically attracted to Historia, 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 Historia. Also at the series panel for Animagic 2014, producer George Wada confirmed that Ymir and Historia are a couple.
- Since Ymir was 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.