Fairy gender will be used for concepts native to or characteristic of the fairy realm, and human gender will be used for concepts native to or characteristic of the human realm. The neutral gender of course will be used for more neutral terms.
For example, there could be a word, which, in the neutral gender means "person" in a general sense, in the fairy gender means "fairy" as in "a person of the fairy realm," and in the human gender means "human" as in "a person of the human realm." Similarly, you could have "feline," "cat" and "fairy cat" respectively*.
It wouldn't always be that literal though.
The actual differences between words will take the form of a vowel change (like words ending in A vs. O in Spanish), but I also want there to be subtle changes to certain consonants. For example, certain specific consonants might be voiced or unvoiced depending on gender, or even change location of articulation.
I'm also considering having a "demonic" gender for words for demons, monsters and other concepts related to evil, hatred, ugliness and the like, but I'm not sure.
* What's a fairy cat? A cat with fairy wings, of course!