Else is an adjective meaning "other" or "different" or "more," as in the statement "She wanted something else." Else can also be an adverb (Walk carefully on the ice or else you will slip), but it appears most often in compound pronouns such as somebody else, everybody else, and who else. In such constructions, the possessive is formed by adding an apostrophe and s to else: "She is someone else's girl." The construction is not entirely logical, but it is idiomatically correct; never say or write "someone's else," "everybody's else," or "anyone's else." However, the possessive form of who else can be written as who else's or whose else but not as whose else's: "Is this hat yours? Whose else (or who else's) could it be?"
Dictionary of problem words and expressions. Harry Shaw. 1975.