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Ymir is a fairly tall girl with black hair swept to both sides of her head. She has small, piercing eyes and freckles around her cheeks. Ymir typically wears the usual Attack Junior High School uniform, but also has been seen wearing a plain white shirt and dark pants with suspenders.
Considered by several to be a tomboy and a bit of a bully, Ymir has no issues with mocking and belittling many of her fellow Year 1 classmates. She does, however, show respect to those who are willing to change negative aspects of themselves and even offers to help them, although this is rarely seen.
- Krista Lenz - Ymir considers Krista to be her best friend and is rarely seen far apart from her. However, she has noted Krista's bubbly personality can get to be aggravating at times. Despite Ymir mocking her like other classmates, her criticisms have been less harsh and direct. While she is rather protective towards Krista, she held no shame into slapping her and telling her she'll get fat if she eats too much. After their brief fight, Krista mentions how they're the only couple, implying that they're in a relationship.
- Bertolt Hoover - Initially, Ymir had no respect for Bertolt and called him a coward several times. She does briefly admire his new-found confidence when trying to help him get Annie's attention during the summer festival, but goes back to scorning Bertolt after he returned back to his old habits.
- Reiner Braun - Ymir appears to dislike Reiner due to him often flirting with Krista. Despite this, they worked well together to help Bertolt confess to Annie.
- Frieda Reiss - Ymir was at first wary of Frieda, even assuming that she was a child molester because of her actions towards Krista. It turned out that she knows of her identity about being Historia's sister, and called her "big sister" in a somewhat mocking way.
- Annie Leonhart - While they have no actual interactions prior to their fight over the purikura pictures, Ymir knew of Annie's name. When Krista proposed them to go take a few purikura pictures, Annie declined, yet Ymir told her that they'll go, before grabbing her collar and yelling at her face. She then tells Ymir that she'll die instead, and as she leaves, Ymir asks if her saying was a joke. According to Annie, Ymir had never acknowledged her existence until her and Krista fought.