In the Manga Appears once in the manga during a special chapter which occurred after the events of the main story. She was shown in a single panel serving Sven and co. as a waitress.
In the Anime Tanya is a waitress that Sven met once before. By pure coincidence, Tanya constantly switches jobs and moves to different cities at around the same time that Train, Sven and Eve arrive at that same city. She is a constant source of free food for Train and company, as she allows Sven to put most meals "on his tab" and gives bread crusts (sometimes deep-fried) to Train. She usually calls Eve Sven's daughter.