Empress-consort rather than empress-regnant, I’m afraid. She was Julia Domna, wife of emperor Septimus Severus and accompanying him on his attempt to bring the north of Britain under his control
That said, there absolutely were empresses-regnant of the Byzantine empire, and there’s no reason to consider that a separate entity. Irene Sarantapechaena and about four or five others absolutely were ruling Roman empresses
TIL. Did the Greeks get less patriarchal over time? In the classical era they were Taliban-tier and complained they even had to see women.