English equivalent of the Hindi phrase "छुपा रुस्तम" (chupa rustam)

You can use dark horse.

dark horse (Noun) A person about whom little is known, esp. someone whose abilities and potential for success are concealed:

"a dark-horse candidate".

A competitor or candidate who has little chance of winning, or who wins against expectations.

The phrase that comes to my mind is, “There's more to him (or her), than meets the eye.”

One website describes this idiom to mean:

Meaning of more than meets the eye: You can say there's more to something than meets the eye if it's more complex, more important or more interesting than it seems at first.

The phrase can be applied to a situation, but, when applied to a person, it generally means that that person “has more to them” than one might initially assume, particularly when it comes to nobler qualities, such as intelligence or helpfulness. It doesn't necessarily pertain to cleverness, although it certainly can:

Ed: Mike seems rather slow sometimes.
Ted: Oh, don't underestimate Mike! There's more to him than meets the eye.

I would refer to such a person with an attributive phrase such as a "real Forrest Gump" or a "'Wrong Way' Corrigan".

The chess term, "gambit player" is perhaps as close an equivalent as you will find, although it implies deceit.

Perhaps the easiest phrase, though, is the simple "wise fool" that I have heard several times. Although it is referred to often as the literal translation of "sophomore", when used in a non-translating context it usually implies the fool who is wiser than he seems.