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 intelligent Titan 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 others with the power of the Titans, 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 intelligent Titans show. Prior to eating Bertholdt Hoover and Reiner Braun's friend, her Titan form was noticeably thinner, and her rib cage was more pronounced, her teeth were also straight and not sharp. 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.
Over sixty years ago, in a residence outside of the Walls, this nameless, homeless, girl lived in the streets as a beggar. Eventually, she was found by a man who gave her the name "Ymir", and brought her into a cult dedicated to "Lady Ymir" who held the "blood of the king". She was worshiped by many people in the cult, and Ymir chose to abide by her new role in order to bring happiness to those around her.
However sometime later, the cult was discovered by a force of soldiers, and the man who named her claimed she had deceived him. Ymir chose to abide by her given name hoping to save the other members of the cult, but this proved fruitless and she was arrested with the other cultists. After being paraded around town and stoned by angry citizens, the cultists were taken to a remote borderline. Despite Ymir's pleas for mercy she and the other cultists transformed into Titans by an unknown method. However since a syringe could be seen in one soldiers' hands it appears they were injected with an unknown serum. Ymir ventured through the country outside of the Walls searching for humans to consume. Eventually Ymir fell into a ditch and was slowly submerged in the earth as it settled over her and would remain in this state for over sixty years.
In the year 845, Ymir sensed Reiner Braun, Bertholdt Hoover, Annie Leonhart and Marcel as they were venturing to the Walls, and emerged from her imprisonment driven to feed on them. Ymir tried to consume Reiner but Marcel pushed him away as Ymir grabbed and consumed him instead as Reiner and Bertholdt watched in horror. Later that night Ymir somehow reverted back to her human form as a result of consuming Marcel in a desert, and when she saw the beautiful night sky she laughed for the first time in years. Eventually Ymir found her way to the Walls and became a street thief within the interior.
Some time later, while scavenging for food in a church she overheard a conversation between members of the Church of the Walls, regarding an illegitimate child of the royal family who was given the name "Christa Lenz" and sent off to the Cadet Corps. Upon hearing one of them express annoyance at her very existence Ymir felt pity for Christa, sympathizing with her position. In the year 847 Ymir enlisted in the 104th Cadet Corps to find Christa.
In the year 849, 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.
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 herring 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 transformation into a Titan.
Ymir fights the Titans strategically, aiming for the nape of their necks and showing great focus. However, while being fast and vicious, her Titan form is no larger or stronger than most Titans. They attack viciously and Ymir grabs at the wall of the tower but releases it as the bricks come loose and the heavily damaged structure 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 on. The tower completely collapses, burying the Titans under the rubble.
When the Titans begin to emerge from the wreckage, 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 are saved by the timely arrival of Hange's squad, which proceeds to slay all the remaining Titans in the vicinity. In the aftermath, Ymir lays almost unconscious in her human form, her right arm and leg missing, with Hange and Christa by her side. Before Ymir passes out, Christa reveals 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 scrambled eggs and claims that any normal person in her condition would be dead. Historia speaks 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 could be a treasure to humanity.
Shortly after, Reiner and Bertholdt reveal themselves as the Armored and Colossal Titans. The two transform and Reiner tries to kidnap Eren who transforms as well and begins fighting him. Meanwhile, Bertholdt partially transforms into the Colossal Titan and grabs Ymir and another soldier. He swallows them both and then helps Reiner. Together they escape with Ymir and Eren.
Afterwards, Reiner and Bertholdt arrive at a forest and take a break high in the branches with Ymir and Eren on one branch and the two of them on another. Ymir waits for her limbs to grow back while Eren regains consciousness, missing both of his arms. He is impatient and wants to fight his way out, but Ymir pragmatically points out that he does not have much of a chance. They are surrounded by Titans at the base of the trees and Reiner and Bertholdt are the only ones with omni-directional mobility gear.
Ymir asks Reiner why he stopped rather than going all the way to Wall Maria, but he does not give a direct answer. Suspecting the real reason, she asks him if the Titans around them move at night like the ones at Utgard, and he tells her that she should already be aware that they would not.
As they wait for sunset, Reiner begins talking strangely about promotions and how hard they have been working. When Eren calls out his behavior, Ymir realizes that Reiner has fractured his personality to cope with the guilt over being friends with the people he is trying to kill. She mocks him and her words clearly hit home. Though she stops pushing Reiner, Eren does not, and when he starts yelling at Reiner and Bertholdt she tells Eren to stop because she needs to be able to count on him.
Ymir asks if Reiner and Bertholdt are trying to meet up with the "monkey" they saw back at Utgard Castle, since they had quite the reaction on seeing it. Perhaps it could even be something they need in order to return home. On hearing this, Eren realizes that Ymir knows much more than he does, and pushes her for answers, but before she can say anything, Reiner offers her a deal. They cannot promise her safety, but they are willing to work with her in order to protect Christa. When Eren asks her who their true enemy is, Ymir simply says, "Who knows..."
While the sun is still up, Reiner and Bertholdt prepare to move out without explanation. Bertholdt asks Ymir if she remembers who she ate when she regained her human form. She admits she does not and apologizes when she realizes that it was probably a comrade of his, given the timing of when it happened. Bertholdt is surprisingly understanding, saying that he similarly does not remember. He asks her how long she had been wandering as a Titan and Ymir says it was about sixty years, like she was stuck in a nightmare that would never end.
The group of them flee using their ODM with Reiner carrying an unconscious Eren and Bertholdt carrying Ymir. As they leave, Ymir looks back and sees the signal flares announcing the arrival of the Scout Regiment. Believing that Christa is too good-natured to not be among the rescuers, Ymir tells them this is their chance to grab her. Reiner refuses, saying that their chances of succeeding are too low, but Ymir fears she will be eaten by another warrior before that happens and she will never have another chance to see Christa. Bertholdt and Reiner try to reassure her that they will rescue Christa later, but Ymir wonders if accepting that means she will be lying to herself again.
Her mind made up after recalling her past, Ymir interferes with Bertholdt's maneuvering and threatens to take Eren back to the Scouts if she does not get her way. She might be the worst kind of person, but she is willing to risk Christa's future if it means they can meet one more time. She can buy Reiner and Bertholdt time to escape, or if they want, they can fight to the death right now.
Ymir takes on her Titan form and meets the pursuing soldiers alone in the forest, where Conny recognizes her and tells the others not to attack. She looks around at the faces of those present until she spots Christa, who she captures in her mouth. Ymir then turns around and swings from tree to tree to escape. Reiner and Bertholdt are waiting for her at the edge of the forest. On seeing her, Reiner transforms into the Armored Titan and begins running while Bertholdt grapples on to his shoulder with Eren. Ymir jumps on his back and he carries them all away.
As they flee, Ymir removes Christa from her Titan's mouth and partially emerges from the nape. She calls her Historia for the first time and apologizes for eating her. Ymir tells her that she is going along with Reiner and Bertholdt, and this is for Historia's safety as well. It is not as bad outside the Walls, and no one will say she should never have been born.
Historia does not believe Ymir and offers to help her fight off Reiner and Bertholdt, saying that no matter what she will always be her ally. Though Ymir wavers, she decides she cannot let Historia go. Ymir expects she will be killed to retrieve the Power of the Titans she stole, but if she hands Historia over Reiner and Bertholdt, they will speak on Ymir's behalf. She calls herself pathetic for doing this to save her own life, but Historia reassures her that she will still be on her side.
Commander Erwin Smith leads a horde of Titans into Reiner to stop him and Ymir is forced to defend Historia. In the chaos they are separated from Reiner and Bertholdt, but they manage to kill a Titan together. As the battle intensifies, Ymir questions whether she is better off siding with the warriors or the Scouts, but any way she looks at it, the chances inside the Walls are bleak. Historia tells Ymir that despite all her actions, she believes that Ymir is still trying to protect her rather than living for herself.
They join Sasha and Conny in fending off the Titans around them when a newly freed Eren strikes a particular Titan with his bare hand, triggering something that everyone with the Power of the Titans can feel. This causes all the mindless Titans present to drop what they are doing in favor of devouring the one that had attacked Eren. As he runs away he screams and sends the Titans after Reiner and Bertholdt.
Ymir now realizes why they were after Eren, and that with his power the people of the Walls may have a future. The Scouts prepare to retreat during the confusion and Ymir looks back to see Reiner and Bertholdt are about to be overwhelmed. She reaches out to Historia and caresses her hair before apologizing and turning to run back. Ymir arrives in time to stop a Titan from eating Bertholdt.
With her assistance, the three make to back to Wall Maria where Reiner asks her why she came back to them. Ymir calls herself an idiot, but that she will be a souvenir so they do not return empty-handed. If they had not come to break the Wall, she would never have woken from her nightmare, so she is going to return what she borrowed. Bertholdt shakes as he thanks her and apologizes. With tears in her eyes, Ymir reflects that it was not so bad being a goddess for a while.
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 Reiss - 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.