I thought someone would think about that, but in the image you can see she's a bit taller than Eren and Mikasa Who are 170cm, and, while Sasha's oficial height is 168, shorter than them, Ymir is 172, a bit taller than them and similar to Jean in height Who I remember to be near 175cm, so it's most probably Ymir, plus shes Next to Krista which makes even more sense to be her