She's every NTR Trope on legs and I hate it D:
I've been trying to figure out exactly which x-dere this is. It's not sadodere because it's not really bullying. It's
close to Hiyakasudere because it is teasing, but this is particularly nasty teasing. This is intentionally making your partner think that he's about to be cucked. It sounds like it's between those two tropes and then at right angles add the NTR threat aspect. Pretty nasty and not what I would consider "playful".
Hiyakasudere:
"They will act really seductive in front of other people, mainly those that attract their attention, and will tease those that get embarrassed easily by their actions and words, possibly the love interest."
...
"
Hiyakasudere is also much like
sadodere, who like to toy around with their love interest like a
hiyakasudere does. However, a
hiyakasudere does not manipulate or wants to cause tremendous pain. They only do and say things to playfully infuriate the person they like or leave them flustered. They are not sadistic as a
sadodere is."