Wikipedia says, "an archetype of a kind, unassuming, and honest girl or woman who lives near by, often in a romantic story". That excludes most prossies.
The next thing is what do we mean by it in a prossie sense?
For me, it would be Scottish (or at least English without a plummy accent), a little bit of make-up but not too flashy, dressed smart but very casual, good-looking but not self-consciously so, relaxed, unhurried, quietly spoken, medium build, between 19 and 27, has at least the appearance of a proper job, can talk in complete sentences, makes natural eye-contact, can smile without it looking false, acts like she's not been in selling her fanny more times than the local chippie has sold fish-suppers, uses the name I've given her rather than going "darling" "baby" or generally talking like a stripper at the pub.
There's few that fit that description -- probably none -- but the ones that have come closest have been the best of the students when they were around -- AlexaF -- one or two others came close, and one other well-regarded prossie in Edinburgh who I am not mentioning again as she gets enough publicity already.