The Master of Disguise is a character who can slip in and out of costume at the drop of a hat, who is such an accomplished actor and makeup artist that no one can identify them until they reveal themselves. This trope is just about the "accomplished actor" part. When they want to, they can change their mannerisms, their voice, and every other non-physical aspect to seem like a completely different person. Usually done by evil characters, in a Bitch in Sheep's Clothing sort of way. This is the kind of character that causes fans to cry "wait, is that even the same character?"