The Qowat Milat were an all-female order of Romulan warrior nuns. While a male could train in their ways, he could never truly be one of them. The Qowat Milat were the most feared enemies of the Tal Shiar.
The primary teaching of the Qowat Milat was the Way of Absolute Candor, which was the total communication of emotion without filter between thought and word. This was the opposite to everything that mainstream Romulans held dear. (PIC episode: "Absolute Candor")