
I've seen this kind of plot device in yaoi countless times. The fact that it's not unique makes it all-the-more far-fetched. If you're going to create an antagonist, it's recommended you do so such that her actions seem believable.
It's either that or there's just an obscene number of women in the yaoi universe with schizophrenia. Like 80%. No wonder the men 'turn' gay ('cuz in the yaoi universe, apparently you can just rotate sexuality the way you rotate tires...)

true....one of my friends(we don't talk anymore but we follow each other's social media) that would purposely befriend guys that her boyfriend knew got feelings for her and she will act in class(I'm in different college, one of my best friends is in the same class with her) as if her bf is trying to control her social life because she has more guy friends because she say and I quote, "Boys have less drama".
but in reality she "broke up" with him when he was talking to a female colleague (he's not in college) that looks prettier than her.(my "friend" is not bad looking. she's just too...lets just say she's a two faced bitch) And they later got back together when the bf(the bf is really in love with her fyi) agrees to not talk to another girl without her permission.
in High school she would tell us about how a girl she doesn't like is too slutty(she prettier and more popular then my "Friend") She later on befriended her to gain some of the girls popularity. My friends and I were so confused.
at the end of the year, we finally notice that she would backstab and lie to anybody just to gain their liking of her. including us. she has talk shit about us to each other for years. (She would talk to A about how she doesn't like B, and would talk to B about how she doesn't like A, etc etc. her lies include on how my friends didn't have dignity because they're in several relationship throughout high school, how one of us dress slutty, how ashamed we should be for talking to guys more than girls[irony, right?]) and At the end of high school we agreeded on never talking to her again unless it was necessary.
tl:dr: girl is a lying , manipulative bitch.
this here ends my rant about a bitch who switch her personality based on the people she talked with.

You know that friend of your reminds me of a past friend of mine,it's sad really this may seem far fetched but in fact its a constant reality of the female populous and their search for the ultimate package. Cutting the throats of all those that get in their way but the good thing is that all women aren't like that......

Pardon my intrusion into this conversation, but I wanted to add a little something. I'm guessing that I'm at least 10 years older than you both, if not 20 or more, and even I have had experiences with females like the ones you've both described. So, this behaviour is not limited to your generation. It's been going on since we "climbed out of the trees", as they say. Yet, funnily enough, men are the ones who are considered to be competitive. I think that idea is ludicrous. Women are FAR MORE competitive than men. Not all of them, mind you, but enough that it makes the rest of us look bad. You know what they say, "One bad apple can spoil the entire barrel." and "Only 10% of the people you deal with are trouble makers, but you deal with that 10% ninety percent of the time." Women like those that we are discussing like it that way. It's the attention they seek, regardless of whether it is positive or negative attention, since even being hated will garner them sympathy from someone - usually those who don't know them well. In my experience, the best way to deal with them is just as you both have done, ignore them. Give neither sympathy or hatred. Without the attention, they lose interest and allow you to live peacefully.
Thank you for allowing me to add my "two cents". It was nice talking with you and I look forward to doing it again.
I dunno why I keep reading this Mangaka's work. They never go for full drama I feel. Always being half-assed. Plus, they have a knack for making their semes way too aloof and bipolar.