Ymir came from a family of immigrants from Africa to live in Brooklyn, where she grew up. When her parents fled to Rapture to escape the discrimination of the people on the surface and were in desperate need for a job, she ended up living in separate homes in Pauper’s Drop and Siren Alley because her parents had to work separate jobs. She was eventually taken from her parents by Fontaine’s men and was constantly abused by them. Ymir had been raised as a little sister from Rapture's Fontaine Orphanage for Girls. She of the oldest girls there and was quickly taken by Dr. Hanji Zoe to become a Big Sister. She was one of the first Big Sisters inaugurated into the Big Sister program. She was given special tasks to go to the surface and bring more little girls to Rapture to turn them into little sisters and even assassinations under orders from her superiors. However, there is one task that she had been given. This task required to bring a certain “special” girl to Rapture so that Fontaine and Petra Ral can turn her into a little sister. This special girl was not just an ordinary girl and was not located on the surface land. This job she was given was to take this girl from a city in the sky…. And drag her into the sea.

As a big sister, Ymir at first speaks through screeches and body movements. She mostly speaks through flashbacks and audio diaries for most of the series in Attack on Rapture, but soon gains the ability to talk through a radio transmitter built into her suit. Her voice is tomboyish and, like Mikasa and Annie, has a somewhat cold sounding voice and dark humor. However, she has a bit of a more gruff and tough edge to her. She is more comfortable showing her personality and emotions around Krista and she is less harsher than Annie. She has a bit more humor to her and a rough attitude, but is still cold when interacting with other characters besides Krista. She also has to either have a thick Brooklyn or African accent.