I was gonna argue that Yates is more of a Gryffindor than a Slytherin, but he’s more of a Slytherin than Jo or Liz or the Brig or Benton, so I won’t complain.
Still, if he had a choice, I think he’d pick the bravery and chivalry House. Though he’s very loyal and that can be a problem for him, too, going too far for loyalty or for what he believes is right. The whole idealist bit.
Slytherin is a little off for him because he’s fundamentally lacking in ambition. His ambition is pretty much… protect the Earth and the people he loves, do what is right. He doesn’t expect anything in return, and doesn’t want anything for that matter.
But then again he is posh and privileged, in his way. Plus, he’s adept at spying and deceit. When brute bravery isn’t enough, he’s the one that UNIT goes to to pretend to be a man from the ministry or a hippy or whatever else they need. He has the cunning to accomplish what needs to get done, and he will do what it takes, even if it means he has to live with himself afterwards.
SO MANY MIKE YATES FEELS. Story of my life