She is referred to as Polonius Lartessa's second. In her human form she is described as lovely, with long dark hair and deep dark eyes. She is quiet, with an air of sadness. Harry Dresden generally seems to think that her sadness is simply a way of manipulating men, but it may very well be genuine. Rosanna also has magical ability, but likely not on the same level as Tessa. She and Tessa seem to be genuinely close.
Rosanna recruited the Russian Sanya for the Denarians. She offered the sixteen-year-old Sanya the coin of the Fallen Magog, and afterwards travelled with him for at least five years, in truth as his handler in charge of corrupting him. After realizing their true intentions, Sanya left her and abandoned Magog, dropping his coin into a Venice canal.