Fairies

Ouji Misaya's familiars.
Originally, fairies are considered the nature's sense of touch, something that humans cannot perceive.
However, there are indeed some fairies who were born wearing humans' imagination as shells.
Ouji Misaya's Familiars are false fairies. They are formless low-rank Spirits possessing the image of woodland fairies.
They can indeed wield a portion of the original fairies' powers due to wearing fairy-like shells.
The fairies-disguises are thought to be the creation of Misaya's Master rather than hers.
As an aside, true fairies at the scale of capable of being perceived by humans are called Elementals. A certain white vampire seemed to be of this class.
One more thing, the foundation of fairies and elementals are of a mystery that cannot be reached through magecraft.

Garden of sinners Pamphlet: Kara no Kyoukai Settings Glossary