In this case, the question (Who?) is about the subject. "Who lives here?" "Mark lives here." (compare "Where does Mark live?" "Mark lives in France." i.e. the answer is not the subject but the indirect object - extra information about the subject). Other subject questions: "What has 8 legs and lives in the ocean?" - "An octopus...".Good morning.
What about questions such as "Who lives here?"?