Consider the word
The most famous gorgon is Medusa. often mis-portrayed in recent films, Medusa actually has tremendous beauty; the curse was to add this hideousness that caused death to mortals. All the Gorgons have this quality.
It's maybe 'leveraged', but you could think of any beautiful thing that has a surprise-ugliness as a gorgon.
Since gorgons are powerful creatures, this term could be appropriate to the heroine of a story.
FWIW, I do not think there is a non-derogatory word for this, since the judgement of ugliness is inherent in all such terms. If you want a non-judgemental, the closest I can think is
I.E.: Appears ugly, but has a hidden beauty that emerges.
But, this is the opposite of what you asked, which is that the beauty is seen first, the ugliness later. By compare, Medusa's beauty was recognized by mortals only after she was dead.