|I'm Ymir, and if any of you scumbags lay a hand on my Christa, you're dead meat!|
|— Ymir, protecting Christa|
In company with her low ponytail, bangs-parted hair and freckles, Ymir also wears the same uniform as the girl students of Junior High, only she keeps her blazer open at all times.
Right from the beginning, Ymir focuses on chasing away boys that have a clear ulterior motive to get near her friend Christa. She is wary of Reiner, who tries to approach Christa at times. However, Ymir has shown to warm up to specific people, such as Bertholdt, where she tried to help him ever since he admitted his crush on Annie.
Ymir finds Christa fallen on the ground and picks her up, immediately taking a disliking to Eren Jaeger, the person who knocked her down. As they run away, Ymir shouts angry slurs at them. Ymir is met with her classmates at the front of the Attack Junior High School gates, where she runs into the Titan side of the school and enters the Titan class 1-4. She safeguards Christa from falling stationery that the Titans of the class drop. She is escorted by Hannes to the human class of 1-4 where she introduces herself to her class, warning people to not come near Christa. Later, she goes to the entrance ceremony where she protects Christa and her lunch from being stolen by Titans.
Ymir finds her friends in the courtyard complaining about their clubs. She follows her friends to locate the Attack Junior High Scout Regiment, where she meets odd upperclassmen who welcome her into the club, despite being in a different one already.
Ymir lags along with Christa as she asks Annie about her favorite lunch. Along comes the dodgeball match the next day with Ymir's participation in Annie's team. When the ball hits her beloved Christa and Reiner rushes to her aid, Ymir prepares on stance as she trips Reiner and saving Christa from being out. Later on lunchtime, she and the other teammates offer their Mozuku lunches to Annie.
Ymir jokingly hits Christa with a xylophone mallet in the Music Club.
One morning Ymir arrives with Christa and come across Sasha. Christa gives Sasha some food and the latter gratefully embraces her. Ymir separates them, telling Sasha to stay away from Christa. A few minutes later, the three girls notice Jean's odd behavior and Ymir securely holds Christa.
Along with Christa, Reiner, and Bertholdt, Ymir arrives late for the Scout Regiment club activities and thus the first years group is grounded with cleaning chores. As Christa is about to fall from a ladder, Ymir pushes Reiner and catches her. After Levi gives the group a challenge, Ymir participates in the school's Sports Day with her class. She is amazed by Christa's cheerleader costume and gains motivation. For the ball toss challenge, she watches Jean being beaten by the second years but shows no compassion. During the shoulder wars match, she teams up with Sasha, Mina, and Mikasa.
The upperclassmen plan a sudden "Test of Courage" event at night. In the bathroom, Ymir and her group hear one of the school's Seven Wonders, a ghost voice coming from one of the stalls after Ymir dares knocking it three times. They run out of the bathroom, and Ymir is separated from Christa after she mistakenly carries Sasha instead of Christa.
Distraught, Ymir runs around school crying for Christa, until she is reunited with her outside. At the gym, everyone starts running around as ghosts and a giant cannon appear chasing after them. It is later revealed that the event is a prank played by the upperclassmen.
Ymir teams up with Reiner in order to get Bertholdt to ask Annie out at the summer festival. Arriving there, Ymir kicks Reiner when he tries to compliment Christa in a kimono, then Ymir herself compliments her. She tells Bertholdt to say something to Annie, and he compliments a knot on her geta strap, to Ymir and Reiner's disappointment. When Sasha and Jean argue over shaved ice, Ymir sneaks behind them and pushes the cup towards Annie, so Bertholdt could shield her.
Ymir and the others later hide behind bushes and eavesdrop on Bertholdt and Annie. When Bertholdt asks Annie something and she agrees, Ymir and Reiner congratulate him, only for them to discover later that he asked her to do die-cutting.
Following the election for a new student council president, Ymir is present during the candidates' final speeches. When Reiner states his plans for holding swimming meets to increase the opportunities of physical contact between the students, as well as showcasing female students in swimsuits, Ymir glares at him while tightly holding Christa.
In the school's 104th festival, Ymir and Christa participate in their club's maid café, serving Reiner and Bertholdt omelet rice. Seeing the look on Reiner's face, an irritated Ymir spoils his meal. However, he eats it regardless, then asks Christa for another one. When Ymir attempts to spoil it again, Christa stops her, only for Reiner to then burn his mouth since Ymir had secretly put hot sauce on the inside.
When the teachers get in the way of Eren trying to head to the Titans' building to retrieve his cheeseburgers, Ymir trips one of them. She later watches Ilse Langnar's broadcast of Eren and the others venturing into the principal's office, and cheers once Eren manages to retrieve a chee-burg.
- Christa Lenz - Ymir is comically in love and utterly obsessed with Christa. She intends to protect Christa with every fiber of her being, all the while scaring off boys who try to flirt with Christa. She is snide to everyone besides Christa, who returns Ymir's appreciation.
- Reiner Braun - More often than not, Ymir keeps an eye out for him due to the fact that he seems interested in Christa. Ymir disapproves of his behavior towards Christa, but overall thinks that he is a loyal person to the people he is close with, such as Bertholdt. Before the summer festival, Ymir even teams up with Reiner to ensure that Bertholdt gets Annie as a date.
- Bertholdt Hoover - They are classmates and get along relatively well. Ymir quickly notices he has feelings for Annie and tries to help him out with it. She even teams up with Reiner to make plans so that Bertholdt can get Annie's attention.