Appearing solely in "Victoria" and (largely) in the shadow of her daughter Princess Victoria, this British monarch is the epicenter of events swirling around Gerard, Chance and the team. Her heir has decided to terminate her loveless union to Prince Walter and go instead for an NY paramedic, one Tony Sorrento. This sends the family's fixer, Templeton, into a tailspin, which all comes to a head when Victoria survives several assassination attempts and confronts him in her mother's presence. Since we last hear of Victoria on her wedding day to Tony, presumably Mum thawed somewhat on the subject of sons-in-law, or at least allows her child's happiness to outweigh her doubts.
She is presumably the in-universe equivalent/manifestation of Elizabeth II.