@Lynkaster Sorry to tell you, but the way I read it, the "spoiled demon princess" is indeed her own self. Prim and proper is a mask that she can barely keep up at work, fun and lively she can handle with (boy)friends as long as they keep up with her every demand (and go along with whatever story she's made up in her own head).
But I do agree that I like her - as a rival, since the main couple need something shake them out of their current rut.
(And if this had been another manga I'm sure rival girl would have been redeemable for MC - but within the bounds of this manga I'd cheerfully point her towards cafe owner who might need some of her energy while being able to impart some adultness upon her