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Although a sidekick is good, some such characters are called a "foil", meaning one character that is in some sense the opposite of another, and thus highlights a trait of the MC. In your example,...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/40353 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/40353 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Although a sidekick is good, some such characters are called a "[foil](https://en.wikipedia.org/wiki/Foil_(literature))", meaning one character that is in some sense the opposite of another, and thus highlights a trait of the MC. In your example, the "opposite" trait may be knowledge of what is going on, or ignorance vs. insight. The villain explains his plot to the beginner, or assistant, or dim-witted muscle, or know-nothing girlfriend, or clueless but wealthy financier. A sidekick is usually with the protagonist (or antagonist) for most of the story; a foil can be used for just one or two scenes (like a girlfriend, or the financier considering funding the antagonist).