The lexical definition of the word "person" does not have a lexical meaning with a scope sufficient enough to include these non-existent entities (which isn't to say that a non-existent entity is a kind of entity, of course) that you refer to as imaginary people. Yes, we can disambiguate contexts by explaining what kind of things we are talking about when using ambiguous words; for instance I can qualify the word, "right" by contrasting moral rights with legal rights, and if you want to use the term 'imaginary person' and stipulate a meaning, that's fine, but lexical meanings are a function of how fluent speakers of a language collectively use a term, not to be confused with what people are trying to say, as what one may mean by any particular term has no bearing on it's actual meaning. Also, it would be highly advisable to dismiss this notion that imaginary people are somehow located amidst electrochemical brain activity. There is an unfortunate tendency for people to ascribe location to things that are without location, and it's particularly odd to ascribe location to things that clearly don't exist in any form or fashion.