Dissembler, n.
One who dissembles; one who conceals his opinions or dispositions under a false appearance; a hypocrite. It is the weakest sort of politicians that are the greatest dissemblers. Bacon. Priests, princes, women, no dissemblers here. Pope.
Syn. -- Dissembler, Hypocrite. A person is called a dissembler with reference to his concealment of his real character, and a hypocrite with reference to his assumption of a false character. But hypocrite is the stronger word, being commonly used to characterize a person who is habitually insincere and false, especially one who makes professions of goodness when his aims are selfish and his life corrupt.
Shades of meaning
Fernald has no entry of its own for dissembler; he lists it under hypocrite.
Where he writes about it:
The deceiver seeks to give false impressions of any matter where he has an end to gain; the dissembler or hypocrite seeks to give false impressions in regard to himself. — from hypocrite
The dissembler is content if he can keep some base conduct or evil purpose from being discovered; the hypocrite seeks not merely to cover his vices, but to gain credit for virtue. — from hypocrite