Once upon a time, the Slave of the Magic Mirror was, in life, Queen Grimhilde's father, a poor Mirror Maker who was always rather harsh with her, as his wife died in childbirth having her. Years later, after his death, his soul was captured by the Friends and reconfigured into a 1st Class Agent, a spirit of the darkness forever imprisoned in the Mirror.
Through this weapon, the Wierd Sisters, masquerading as relatives to King Leopold, were able to use it as a means of leading the Queen to a darker path that would eventually lead her into allegiance with the Friends on the Other Side. Part of this reason was that as long as Grimhilde was satisfied at her father's ghost finally calling her "Fairest in the land" after years of saying otherwise, she was confident in herself. As for the Mirror Maker's soul, he was merely indifferent in his lot in undeath, serving out the role the Friends gave to him with the utmost loyalty to his daughter.
After Grimhilde's demise, the soul of the Mirror Maker remained trapped in the mirror all alone until the wizard Yen Sid obtained it in his travels across the worlds. Although the Spirit of the Mirror was at first unwilling to serve a new master, he was eventually convinced otherwise when the wizard confronted him with what he did in life and that now was the time to break free from the darkness to be with the light. The slave in the mirror thought long about this, but eventually decided to remain a neutral party as the fact remains that he was still a transfigured Agent of the Friends and most likely knew too much of their secrets to let slip so easily to the High Council.
These days, the Slave in the Mirror serves as a neutral oracle to either party in the war, giving out information to those he deems worthy of it.