Having thought a bit more about it I realized that having the fairy sort of gently puppeteer you around as her mount/plaything or such could be really nice. Something like using some sort of telekinesis, telepathy, strings or just words to make you move to her will in a way that requires your cooperation(because there are already characters for forcing you to move exactly as they want so more gently doing a similar thing seems fitting and the fairy complaining and/or magically whipping you when you fail to cooperate sounds nice to me). Could even turn her into major character with a relationship meter but just having it as one or two time thing would be fine. If a fairy became a more major of character question of how would they accompany you if and when they need to is an interesting one, fairies typically being somewhat loose about their physical form means that shrinking and becoming somewhat intangible and going into your ear and possibly further in is a potentially fun option though just hiding in your hair or such would be a more natural way of discreetly staying with you, unbirth is also an interesting option.
Regardless more events for the fairy tree in whatever form they might take would be nice to see since I ended up feeling like there should be something more to it than just resting there after I unlocked it as location(obviously there already might be something that I just haven't seen yet).