Broke:
Kris is able to flirt because of the soul/player/some outside influence
Woke:
Kris knows how to flirt because they had infinite time to figure that out before chapter 1 even began, as they were able to save and load on their own
Bespoke:
Kris knows how to flirt because Asgore, Asriel, and Burgerpants are so freaking bad at it that between the three of them Kris has seen every possible way to fuck up, and simply doesn't do anything they did