I'm looking for a word (a noun) to describe a person who is really good at something but somehow underrated.
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I've always like the phrase Unsung hero for someone who's contribution is underrated/valued:
Unsung hero: a person who makes a substantive yet unrecognized contribution; a person whose bravery is unknown or unacknowledged
It depends what you mean by "somehow" underrated, but I would refer to somebody like that as a dark horse
a person who keeps their interests and ideas secret, especially someone who has a surprising ability or skill:
Anna's such a dark horse - I had no idea she'd published a novel.
It's marked as British English, so may not be known in the US.