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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.
- 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.