A fairy tasked with helping out the protagonist of a tale, whether it's granting wishes, or other things. Why this is so is almost never said, although some stories will have an organization of them.
This is also a highly Discredited Trope
these days, usually associated with the most archaic parts of Fairy Tales
, even though this trope is a lot more recent than most of those stories
of Our Fairies Are Different
Compare Fairy Companion
, God Was My Copilot