When she was first introduced, her puckish and maniacal behavior was incredible to watch and at the time, many of us didn't know who she was and what her relationship with the Doctor was. It was maddening!
But, with all great reveals, we found out in Peter Capaldi's 12th Doctor's episode, "Dark Water", that she was in fact, The Master, one of the Doctor's greatest foes. She had dubbed her self "The Mistress" and appropriately, "Missy" for short. Her performance and interpretation of the iconic villain was a delight.