Complex sentence: The most notable Time Lord to have appeared in both male and female forms is the Doctor's nemesis, The Master, portrayed from 2014 to 2017 by Scottish actress Michelle Gomez; the character was known as Missy, short for "Mistress".
Split into simpler sentences: The most notable Time Lord to have appeared in both male and female forms is the Doctor's nemesis, The Master, portrayed from 2014 to 2017 by Scottish actress Michelle Gomez. This character was known as Missy, short for "Mistress".