Which Byzantine empress, a former actress, co-ruled with Justinian?

Theodora was the daughter of a bear-keeper at the Hippodrome and worked as a performer, a trade that Roman law treated as disreputable enough to bar her from marrying a senator. Justinian had the law amended. When he became emperor in 527 she was crowned with him, and contemporary documents name her as a partner in government rather than an ornament of it.

The moment that fixed her reputation came in January 532. The Nika riots had burned much of Constantinople, the crowd had proclaimed a rival emperor, and Justinian's council advised flight; ships were waiting. Procopius reports that Theodora refused to go, arguing that anyone who has worn the purple should not outlive it, and that royal purple makes a fine burial shroud. Justinian stayed. His generals then penned the crowd into the Hippodrome, and the reported death toll is around thirty thousand.

Her legislative record is unusually specific: measures against forced prostitution, wider grounds for divorce, stronger property rights for women. She also protected Miaphysite clergy while her husband upheld the Chalcedonian settlement, which looks less like inconsistency than a couple covering both sides of an empire-splitting quarrel. Nearly everything colourful said about her comes from Procopius, who wrote an official history and a scurrilous Secret History, and the two do not agree.