Fairies are often depicted as sweet innocent creatures, often winged and small. Fairies, however, can come in all shapes and sizes and can be far from innocent sweet creatures. They are often tricksters and play pranks, causing problems for others for their own amusement. They are a magically gifted race and can hide their true forms. It is impossible for them to lie, and knowing their real name, the true name, would grant complete control over the fairy.
They're incredibly magically talented, and they can glamor themselves to appear as almost anything they want
They have an inability to tell lies, and finding out their soul name gives the person who found out a dangerous amount of control.