The purely functional purpose they would serve that I'm pretty much set with is:
1. To offer alternate internals while Amelia is the only prey (will offer the ability to fuse with the fairy, totally optional and can be ignored if not interested. Would offer some small bonuses, but nothing required.)
2. Unlock a different part of the game and incentivize that Amelia is prey long enough to collect 'essence' from each predator, which will be needed to unlock the final boss.
3. Also as an optional support for Amelia. Fairies will only become Amelia's prey, and while prey to Amelia they can have commentary and also update on the progress of essence collection. The fairies will be tiny, so this won't affect the external images at all as Amelia's stomach will stay flat.
I'll add an option to mute/ignore the Fairy for anyone who might not be interested at all in them. Having Amelia eat one will be required to beat the game, but beyond that I'll try to keep it pretty flexible just in case some find it bothersome that Amelia isn't 100% prey.