The dancer/priestess girl is certainly Müllenkamp, but the symbol on her back is actually the Iocus Rood, the Cross of St Iocus, rather than the Rood Inverse.

The Iocus Rood is the symbol of the Iocus Priesthood, Vagrant Story's equivalent of the Christian Church. The Rood Inverse is a 'dark' version of the Iocus Rood, branded on the backs of heretics by a Spanish Inquisition-style branch of the Priesthood. The most notable feature of the Rood Inverse is that its central piece is a dagger point downward, unlike the conventional Rood.

Müllenkamp should have the Rood Inverse, since she was one of the heretic cult leaders, but instead she was rendered with the Iocus Rood. This is an error, I believe, but it doesn't matter a whole lot. They just drew design with the "light" cross rather than the "dark" one.

This is the Iocus Rood, the holy symbol that's throughout the game and also on Müllenkamp's back; I couldn't find a good picture of the Rood Inverse, as worn by Sydney Losstarot.