The Master's gender is "cat!"
I had a very entertaining dream this morning in which the Spymaster was earnestly discoursing to a bunch of men. He was trying to convince them that the gender of [domestic] cat was superior to their gender. His audience consisted of men who subscribed to the current form of patriarchal masculinity celebrated in modern American and British mainstream culture, so they considered themselves powerful, rational, strong, dominant, and righteously in control compared to other genders.
He was actually arguing that being a [domestic] cat is better than being a man too: "People PET me and FEED me and do WHATEVER I want..." As he put it, he had all the benefits of servitude and obedience without any of the worries. His Cheetah Planet biases were showing...
Well, we always knew that the Master doesn't do gender the way that humans do. Of course it makes sense that the Spymaster's gender is "cat." [We just won't mention the DOMESTIC part.]
Speaking of cheetahs, though, has anyone considered the hilarious implications of cheetahs being Very High-Strung And Really Rather Shy AND the Master being a cheetah? For at least fifty years, cheetahs in zoos have been paired with dogs, particularly Labs, goldens, and Anatolian shepherds, and the cats benefit from partners whose playful confidence helps them feel more at ease. Cheetahs can be too nervous to successfully breed, so canine support can actually promote continuation of the feline species!
The prospect of a Cheetah Master who could benefit from an EMOTIONAL SUPPORT ANIMAL is endlessly amusing.
The Cheetah Master's companion animal would, of course, be the Doctor...or perhaps the DOGtor. I'm not sure that the Dogtor would calm the Cheetah Master so much as the Dogtor would inspire the Cheetah Master to act more like themself and vice versa. Their combined insufferability quotient would rise to astronomical levels, and they'd annoy everyone.
But I like to think that they would bring out the best in each other, making each other less self-centered, heedless, and destructive to the universe in general.