If someone is trying to make people underestimate him for some purpose, what would be a word for this kind of person or action?
One term is sandbagging.
From the current Wikipedia entry for sandbagging:
Another term with a fairly specific use is hustle. It's most often used in the context of pool (billiards) players, who downplay their skill in order to attract opponents. Once the newcomer gets comfortable winning games with relatively small bets, the hustler raises the stakes and wins a large sum quickly.
Merriam-Webster mentions the noun form hustler (which is the exact term for the 'someone' in your question), but doesn't have a separate definition it. From Dictionary.com: