Fairies are a tiny, winged race of creatures in the Monster Girl Encyclopedia world that are not naturally counted amongst the monsters. One of the intelligent non-monster races, along with dwarves, elves, and humans, fairies can be corrupted when exposed to demonic energy, turning them into nymphomaniacal, monster versions of their original selves.
A playful and carefree race, fairies dwell in a magical spirit realm called the Fairy Kingdom, where they make little houses out of trees and mushrooms. Even before the age of the current Demon Lord, fairies have always been an all-female race, though they were (and many still are) asexual creatures born via the blooming of "fairy flowers". Their highly magical nature means that, like elementals, they are easily susceptible to corruption, and on becoming monsters their nature changes to sexual creatures who get pregnant and give birth. This does not, however, change their size at all, so sex with a fairy is a "stretchy" affair. Still, they're monsters, so it is exceptionally pleasurable to them and becomes their new favorite game.
Although fairies are a specific species, "fairy" is also used as a synecdoche in Monster Girl Encyclopedia I to refer to all the species of the fairy family: which includes leanan sidhes too, but not pixies (which despite appearances are really imp-family monsters) or kesaran pasaran (who do live in the Fairy Realm but Wandering Scholar classifies them as technically plantgirls).
This race is the most numerous of all the fairies living in The Fairy Kingdom. The fairies are a race that has the form of little girls that are so tiny, their body can be held by a human hand. They are as childish on the inside as they look and they're a cheerful race that loves music and dancing. Originally, they were not called monsters, but those that received succubus energy and changed into succubi are treated as monsters. They come to forests in the human world from the fairy kingdom in order to play with human men. Once they spot a human man, they'll fly around and likely beg to be played with.
However, they have no resistance to monsters. Given that they're childish, they often play with monsters such as pixies and imps who are also childish in the same way and then become monsters due to their demonic energy. They're likely to be taught plenty of pleasurable games too. Due to ideas put into their head by monsters when they play together and instinct after becoming a monster, they know that if they play with human men, they can have more fun and get off a lot more. When they bet to play with human men, it's mostly sex that they're after.
They can service a man by rubbing their entire little body against the penis and aside from that, with their tiny body, it may look as if not even the tip of a human penis would fit in their vagina, but they've become monsters. Their body has adapted for convenient sex with human men and they can make the shape of their tummy swell and receive a penis with their entire body. From the look of it and the powerful squeeze given to the penis, it seems as if they're about to be torn apart, but even in such a state, they don't feel pain and their body won't break. The only thing they taste is sweet pleasure, when semen, which is even sweeter and more delicious than any kind of confectionery, is poured inside, they experience happiness as a monster.
If there's only one, they have a small body and they're weak, so you can catch them and play with them appropriately and then drive them away if you're satisfied. It's also possible to keep and discipline them. But if there's a group of them, it'll be troublesome. They'll swarm over your entire body and you'll end up being forced to play with them until you pass out.
When multiple fairies form a circle and dance, the circle of dancing will faintly glimmer due to the fairies magical energy and a "fairy ring" will come forth. The thing at the center of it will be taken to the "Fairy kingdom" along with the dancing fairies. In the same way, when one is attacked by a group of them and they copulate so lewdly and wildly it's as if they're dancing, a fairly ring will be created and the man under attack will wake up to find himself in the fairy kingdon. The fairy kingdom is full of happiness and one will end up continuing to "play" with countless number of fairies for the rest of their lives, while enveloped in happiness and no worries to care for.
The scales from the wings of fairies form the valuable Fairy Powder