I'm looking for English word[s] that come close to छुपा रुस्तम (chupa rustam), literally "hidden warrior". It's attributed to a person who is very clever but does not appear so. If I remember correctly, it's not used for deceitful nature.
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.
It sounds like the slang term sleeper, but that word has deceitful connotations. It means an outwardly normal-looking person or thing that possesses remarkable qualities. The implication is that maintaining a normal appearance was deliberate, and perhaps even difficult, such as when a vehicle is given a much larger engine than it was designed for. It's a sleeper only if you can't detect the modifications at a glance. It can also refer to a person who is very attractive when they want to be, but can also make themselves unattractive to avoid attention.
Compare sleeper cell, a group of terrorists who carefully conceal their training until they strike.