That androgynous shorthair is wicked. She knows well that MC-kun is weak (if he wasn't, he'd have shut her shenanigans down as soon as she showed up again trying to wedge herself between him and Sara), and so an increase in girls throwing themselves at him means more disruption of MC-kun's relationship with Sara, and thus a greater chance of winning.
And just as you could expect, while everyone relevant (Sara and Kou) is thinking of everyone else (including a certain person they have no business caring for), Flatchest is sharpening her dagger on the last page. Someone said that the axe is looming over this manga--I'm starting to think so, myself: it seems Flatchest is only present because Marimo's editor believes a story revolving around only girls as busty Sara and Kou (even if Sara has relatively sharp eyes; she has the demeanor of someone without them, though) won't sell; it would figure, because many males have been conditioned to think busts like theirs are "excessive," by now--and, together with it, to think someone who could easily pass as male like Flatchest is actually attractive.
The author clearly doesn't agree with that view of busty girls like Sara and Kou--from the doujin to this manga--but Flatchest had to come from somewhere. There was already an imminent arranged marriage--planned by a major obstacle in Sara's mother--to deal with in this story (not to mention Kou's own machinations), but now Flatchest's sheer verticality is here.