Ymir

Milf (ユミル) is a graduate of the 104th Trainees Squad and former member of the Scouting Legion. She is a person with the ability to transform into a Titan, and has knowledge of many of the secrets concerning the true nature of Titans and the history of humanity.

She is a mysterious and complex character, holding her many secrets close. While not completely willing to trust Reiner and Bertholdt, she betrays humanity in exchange for Christa's safety. Little is known about her past or motivations, other than the fact she wishes to protect Christa at any cost.

Personality
Ymir is a complex person At first she seems selfish, cynical, bossy, sly, uncooperative, exploitative and confrontational, but she can apparently be kind, especially to Christa. She has often come down hard on people not true to themselves, especially Sasha, considering she uses proper Japanese instead of her normal dialect.

History
Almost nothing is known about her past, other than a few hints she has revealed. She was once a person of significance, and states that she was hated for her existence and "died for the happiness" of others. At some point, she and presumably others from her people were transformed into Titans. She spent 60 years trapped in this state, wandering the lands outside Wall Maria while experiencing what she described as an endless nightmare. During this time, she appears to have been a regular, mindless Titan.

In 845, she encountered Reiner Braun, Bertholdt Fubar, and Berik as they made their way towards Wall Maria. She attacked Reiner and ended up devouring Berik after the boy pushed his friend to safety. At some point after this, she regained her human form and consciousness through yet-unknown means and infiltrated the Walls. She spent some time within Wall Sina, stealing and spying on the humans within. It was during this time that she overheard conversation between members of the Wall Cult, and learned about Historia Reiss. Her fascination lead her to seek out the other girl, and enlist in the military.

Ilse Langner, a member of the Scouting Legion that resembled Ymir, encountered a Titan that identified itself as one of "Ymir's people". This Titan confused the two women, and bowed before Ilse while referring to her as "Ymir-sama". Ilse's notes on the encounter were later discovered by Hanji Zoe and are the first hints of Ymir's true nature.

Story
Ymir graduated 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 gave less than her full effort so that Christa could have the 10th slot. After graduation, she followed Christa into the Scouting Legion.

Ymir was in the Trost District when the Colossal Titan broke down the gate.

Ymir is one of the Scouting Legion members taken outside Wall Sina by Mike Zakarius and disarmed of her equipment on suspicion of being a potential accomplice to Annie Leonhardt. When Titans mysteriously appear within Wall Rose and the rookies are sent to warn the nearby villages, Ymir recommends to Christa that they run away, but Christa refuses. After the rookies take refuge inside Castle Utgard, 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's letting on. When the castle comes under attack by Titans capable of moving at night, Ymir has a flashback of her and Christa had to help their friend Dazz. Ymir reveals that she was left on the street and had to go around and borrow money. She told Christa that she should try and start a new life instead of trying to kill herself. Ymir left Christa and managed to help Dazz. Returning to the present time, Ymir jumps from the tower and cuts herself, triggering her Titan transformation. She begins to battle the Titans to the shock of her peers.

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, and she is soon caught. The Titans attack viciously and Ymir reaches for the wall but consciously releases it as the heavily damaged tower shows signs of falling over. Christa reasons that with her power, Ymir could have easily escaped alone, and that she is putting her life on the line to fight for the recruits survival. Ymir then breaks the tower, burying the Titans under the rubble. She rescues her friends and is shown to be capable of coherent speech in her Titan form.

When the titans begin to emerge from the rubble, Ymir launches at them immediately and puts herself at risk once again. She is caught by a larger titan and all other titans then begin to tear her body apart and devour her. Her human body is shown to slowly emerge from her titan flesh while in a state of unconsciousness. In contrast to Annie, who managed to transform back to human and then back to a titan again after her titan form was torn apart, Ymir appears to be much weaker at manipulating her shapeshifting powers.

She is last shown heavily injured and drifting in and out of consciousness. Ymir smiles after hearing Christa confess her real name.

Later on however, Ymir, along with Eren, was kidnapped by Reiner and Bertholdt. She regains consciousness before Eren, though both are left recovering from serious injuries and therefore unable to transform. When Eren attempts to fight, she counsels him against this and points out that they are surrounded by Titans and at a huge disadvantage since their captors are armed and uninjured. She asks Reiner to start talking, as he'd promised once Eren awoke.

Realizing they have no choice but to wait until nightfall, the group settles in for a long wait. When Ymir asks for water, complaining about how its been since they had something to drink, Reiner begins to act strangely. Eren becomes angry about this, but Ymir accurately determines that Reiner's mental state has deteriorated as a result of his inability to cope with his guilt.

Recovering from his episode, Reiner extends the offer for Ymir to join their side. She initially rejects this offer, stating that they don't trust her and she doesn't trust them. But Reiner claims they can trust her, since her only goal is to protect Crista and their goals align since he also wants to protect her. Afterwards, Ymir agrees to ally with Reiner and Bertholdt, and refuses to answer Eren's questions.

Reiner and Bertholdt notice a signal flare, and begin preparing to flee the advancing rescue party. While Reiner subdues Eren, Bertholdt confronts Ymir over her past and the fact she killed one of their comrades. She confesses to not having any memory of that time, but apologizes to him for it. Furthermore, she reveals that she spent 60 years living in the wilds as a normal Titan and only regained her mind and human form five years ago. She describes her time trapped in her Titan form as an "endless nightmare".

As they begin to flee through the forest, Ymir notices the signal flares and realizes Christa has come to rescue her. She demands that they kidnap Christa now, and when they refuse she begins to attack Bertholdt, threatening to fight them to the death if necessary. Even though doing so may endanger Christa's life, Ymir states she's a horrible person that wants to see her one more time.

The rescue team notices a light in the forest, and shortly afterwards encounter Ymir in her Titan form. She ignores all attempts to question her about what happened, looking around until she spots Christa. To everyone's shock, Ymir leaps forward and captures Christa whole in her mouth, immediately fleeing into the trees to rejoin Reiner and Bertholdt. After Reiner transforms, she climbs onto his back and the group begin to flee into the wilderness with the soldiers in hot pursuit.

Titan Shifting
Ymir is able to transform herself to a seven-meter class Titan. Her form is the smallest compared to other shifters' forms, with Eren's and Reiner's being 15 meters high, Annie's being 14 meters, and Bertholdt's being 60 meters high. Despite the small size, her Titan form has an above average speed and agility, which makes her easy to dispatch multiple enemies in a quick succession, she also able to climb walls and trees and jump from one tree to another. But again her size had proven to be a problem, since she was easily overran by Titans not long after her transformation at Castle Utgard.

Relationships

 * Historia Reiss - She is great friends with Christa, and 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. Ymir actively works to discourage Christa's martyr complex, and challenges her to embrace her true identity and live for herself.
 * Reiner Braun and Bertholdt Fubar - Ymir agrees to become their ally, and work with them in exchange for Christa's safety. She expresses that she does not trust them entirely, and believes that even if she cooperates with them, she will eventually be killed by their "warriors". When they attempt to flee without kidnapping Christa, she makes it clear she will fight them to the death if they don't give into her demands.
 * Connie Springer - Connie is an antithesis to Ymir; Ymir is wily as Connie is obtuse. Ymir is selfish as Connie is empathetic. Connie has many friends as Ymir only has Christa as her friend. Inevitably they have antagonistic acquaintance, often exchanging verbal barbs with each other. They have repeatedly clashed (one incident almost physically), as Ymir often made him the target of mockery and he called her pejorative names due to her haughty attitude. Even so, it wouldn't be right to say that they are on completely bad terms; as Connie is quick to defend her when she appears before the soldiers in Titan form.
 * Sasha Braus - Ymir is not ashamed to take advantage of other people, especially if they are naive and simple-minded. She initially helps Sasha back to bed after she collapsed in exhaustion, and uses this to make the other girl feel indebted to her. It's shown that she often pushed her chores onto Sasha, but she also was the one to confront Sasha over hiding her real dialect/accent and demand that she not be ashamed of her roots.
 * Eren Yeager - Ymir and Eren had few interactions prior to being kidnapped by Reiner and Bertholdt, with him admitting he didn't know her well at all. They quickly come to mistrust each other, due to Ymir's secretive nature. Ymir is critical of his short-sighted viewpoint and states that she cannot rely upon him for anything, instead choosing to side with the other Titan Shifters.

Trivia

 * Her name Ymir is the very same as a primeval being in Norse mythology who is the progenitor of all Jotun, or ice giants. This could imply a connection to the Titans of Shingeki no Kyojin.
 * The official website mentions Ymir is in love with Historia (Christa)
 * Though she first appeared in chapter 5 of the manga, she is not named until chapter 37.
 * Ymir spent 60 years as an ordinary Titan, but somehow regained her human form and consciousness within the same year that Wall Maria was destroyed. It is not confirmed, but hinted this may have been connected to her eating Berik, a friend of Reiner and Bertholdt.