Yasha Mousekewitz is Fievel and Tanya's adorable and pretty baby sister in the An American Tail series. Yasha only appears in the first few parts ofAn American Tailand is strangely absent towards the end, possibly a goof on the part of the animators. By Fievel Goes Westher appearance got an overhaul just as Fievel's and Tanya's did, however she still appears to be at least a year old, two at the very most (she doesn't appear to have aged as quickly as Fievel or Tanya). She is given a few amusing scenes in Fievel Goes West but she is in no way the main focus in any scene she appears in, and she descends back into obscurity for the direct-to-video sequels as a baby who is always bundled up and sleeping no matter what is going on.
Her personality was actually enhanced in Fievel's American Tails. In the television series she is shown speaking (only one or two words at a time), and in the episode "Babysitting Blues" when Fievel is stuck babysitting her, she proves to be just as good if not better at getting into trouble than he is, escaping Fievel numerous times and almost being eaten by cats.