A dark horse
Definition 1 of 2
Pro Tip 1/3Surprise Element
Refers to someone having unexpected talents or abilities. 
He turned out to be a dark horse, impressing everyone with his piano skills.
Pro Tip 2/3Unassumingly Skilled
Indicates a person who is modest about their skills may have much more to offer. 
She didn't boast, but in the competition, she was a dark horse.
Pro Tip 3/3Hidden Potential
Implies that someone's true ability has not been revealed yet. 
With his innovative ideas, Mark was definitely a dark horse in the company.