I'm not a linguist, but hyphen-compounding doesn't look quite right to me in this context; you usually see that for disambiguation, in compound participles ("moth-eaten"; "hyphen-compounding"), or to cover a few odd cases like common names derived from phrases ("jack-in-the-pulpit"). I think standard English would be to simply treat the modifier as an adjective ("Rational Harry"; "Girl Blaise"; "Death Eater Ron"); nouns often get coerced into their adjective form here if possible, but it's common to see modifying nouns even if no adjective form exists.

As to why it doesn't get used this way in fan jargon... who knows, but fans do tend to share a (mildly irritating) fondness for unusual lexical and grammatical constructions ("I have lost my ability to can"). Probably just a shibboleth thing.