The first generation Mistress was a cybernetically enhanced Rojo. She was killed by her employer Thep Khufan Commander Tersce Tenebrosi after her true identity was revealed and spread throughout the galaxy, quickly arriving at General Warren from Earth, and also to members of the 10 Squad. She first appeared in Dial Z for Zombified. In New Girl, she was killed and said to be replaced at a later time.
The second generation Mistress is a somewhat mutated version of Mercedes Barnes. She wears armor created by Thep Khufan scientists watched over by Dawn. She first appeared in Ledger Recon. Her first mission, where she joined Thep Khufan Commander Tersce Tenebrosi, was in the following episode, Welcome to Chronospect. She is currently still alive.