Here's why:
* It represents a person: "Mister" refers to a male person, similar to how "woman" or "child" are nouns.
* It can stand alone as a subject: You could say, "The Mister is waiting."
* It can be modified by adjectives: "The kind Mister gave me a candy."
However, "Mister" is often used as part of a title (like "Mister Smith"). Titles are usually adjectives that modify nouns, so in that case, "Mister" would function as an adjective.