Agreed. They've always seemed to have some sort of tension about them even back when the Master was a man. From the suggestive dialogue, the odd behavior sometimes exhibited from either side and the fact that there are even some people - like the Rani and Jo, perhaps - in the show and in real life who ship them! From the fans and the cast and crew, it's impossible to deny that something must have gone down when they were younger, almost like how two people loved each other in their teenage years only for one of them to do something like move away and thus, they become distant but the feelings neverwent away. Except in this case, the "moving away" is going insane and thus murdering and manipulating people while attempting to take over the universe and the "loved each other in their teenage years" we've never really seen or heard too much about in the canon universe. But considering the fact that although they've both tried to kill each other countless times, they still consider themselves really close friends that care for and even respect each other. They're just on opposing sides, as Steven Moffat would say.This tension only grows stronger when the Master regenerates into a woman and becomes the Mistress aka Missy.As such, she's adopted a flirty and feminine personality that greater exploits the tension that's already between her and the Doctor, as evidenced by her snogging him, not objecting to him kissing her in the graveyard, suddenly wanting her old friend back when the other Master's didn't seem to really care about that and even calling him her "boyfriend" who "love's her so much" to the Half-Face Man!But at the same time, it's just as you and Missy herself said: There's more going on.Our "primitive brains", as Missy called it, couldn't "comprehend" how "complex" her's and the Doctor's relationship is and I suppose, in a way, she's right. After all, they aren't human and are superior to us in quite a lot of ways, including what they consider relationships, love, friendship, sexual tension, lust and all that. So it's almost like their relationship is a combination of love (i.e. when they were younger and the Master wasn't insane), friendship (i.e. they still consider themselves friends even all they've been through with and without each other), sexual tension (i.e. the suggestive dialogue and odd behavior), lust (i.e. Missy snogging the Doctor without his consent and perhaps some other off-screen moments like when they were younger) and other such things.So in short, yes, some sort of romantic tension is and has always been there but it's notabout the romance and perhaps sex like it would be with us. I sincerely hope we see more of this on-off friendship and tension between them even long after Michelle Gomez retires as Missy and she perhaps becomes a man again.Thank you for listening to my ramblings.And for also allowing me to use this piece in my group.