The thing is, that when the one of the main motif of the plot revoles around a mystical mist (see what I did there), that literally makes every girl that lives in the hotel horny for the MC, even those who wouldn't even think of it otherwise (Julia and Amber most likely, given their status), then the role of a wing girl, wing guy, wing whatever, becomes completely redundant. Chronologically, things with Julia already started to take off even before Kay arrived on the scene, and basically every single girl showed an interest in the MC, by their own or with the mists help, even those who arrived post-Kay, like Laura, Anika and Hannah. Unless the last remaining staff member, (I'm guessing only one more addition to the roster given that there's only 1 person for pool, club, and security jobs right now) will need to be pushed by Kay to show interest in the MC, or if the guest content will include something similar, she is not a wing girl, she might becomes one sometimes in future, but as of now, that notion is simply not true.You can't talk about wing girls without mentioning Veronica from MotH. Especially not in this thread
Apart from that, kinks are different, and I suspect it's not the "wing girl helping you" part but the "wing girl is having action with other girls instead of you" thing. Can only guess of course, but in the end, what's hot for one may be not for the other.
This is why the comparison with Veronica is completely off the mark. If we want to use MoTH terms, she's closer to Diane. I won't say they are identical, because unlike Diane, Kay is rather blatant, not manipulative, and unlike MoTH MC, this one does seems to go along with her willingly, given the lack of actual choices and linear storytelling. Still, you seem to cut her in on a gig you already got on your own, instead of getting said gig that with her help, and that's isn't really good a pay-off. It's like you have ice cream, and someone asks you to if they can taste it, and you can taste theirs in exchange, but unlike that person who only has one, you have several, and they taste several flavors, while you'll only get to taste one. Hence why the whole "I scratch your back and you scratch mine" statement in the game is very stretched and warped as well.
This is where those who doesn't like her content comes from, because when you give up way more than you get, that's not an equal partnership and exchange, and many won't like such thing, especially when it's this obvious, there's no other chance than just simply ignore it altogether, and even the narration tries to make it seem like it's some super great thing, which honestly just comes off as part of the weird pubertant boyis character for the over 20 years old MC.