It was once commonly believed that fairies had multiple souls, and that each of these souls had a different nature, a different personality.
This makes sense give that everything in nature is duel sided. The river that provides fish and water for the crops is prone to flash floods and will take children from their parents.
The forest which is home to food is also home to the wolves, and is a place in which people can get lost, vanishing forever.
Thus wicked fairies such as the one in "Sleeping Beauty" was actually exactly the same as the "good fairies." However, once the negative aspect of her had been brought forth she became temporarily wicked.
2-Fairies rarely fully matured
Although they might be old fairies never truly ever matured. Most of them would always act childish.
3-Fairies were often refugees
From Ireland to Japan many peoples defeated fairy like beings, forcing them into hiding.
In one tale from Ireland a Queen was kidnapped by the fairies so the King declared war on them, sending out men to dig up their mountains, pouring salt into these to destroy the invisible fairy kingdoms within. At last broken the fairies had to surrender.
4-Fairies were enforcers of morality
Fairies were obsessed with morality.
They would go about rewarding what they considered moral behavior and punishing what they considered immoral behavior. Hard work, kindness, creativity, and cleverness were the most important morals to fairy kind.
Many fairies despised wealthy nobles and appointed a number of Robin Hood like figures to rob from the rich and give to the poor.
5-Fairies are grandfatherly or grandmotherly.
Driving their obsession with morality fairies often act grandfatherly or grandmotherly to the many young humans around them. Indeed even a dangerous fairy can be befriended by calling it "Grandfather" or "Grandmother" or by telling it a story about adventure or true love.
Just like a cat may occasionally adopt a
duckling, even some of the most dangerous fairies
will on occasion come to care for a human. Indeed one
of the the most wicked fairies in Eastern Europe would often
adopt a single human family, even while they tortured it's neighbors.
6-There are fairies who are related to vampires.
Some fairies would drink blood, be banished by herbs, sunlight, the cross, etc. Indeed in many places there were fairies which were also the unclean dead that lived among the fairy court.