Empress's bio is available in the minor character docket and is as follows:

Empress is Bianca's poodle. Though, that surely can't be true, considering it barks in Aramaic chants using a dark and unholy register. This demonic voice actually appears in many different animals throughout the series, from rats to horses and birds, all shouting biblical predictions of the future and all referred to as Empress. In the latter half of season one, Empress begins to speak in English but remains just as mysterious in their references. At Bianca's going away party, Empress attacks her and expresses that they have no loyalty to Bianca but rather "darkness itself."