Imps are creatures that were once fairies but have fallen from that state by being kept indoors overnight.
Unlike a fairy, an imp does not have wings. Instead, it has dark, leathery skin, and nubs where the wings used to be. In book one, Seth captures a fairy in a jar because he is inspired by Maddox, a Fairy catcher, trader, and buyer. Unbeknownst to Seth, a fairy kept indoors from sundown to sunup turns into an imp. In retribution for their sister's fall, the other fairies morph Seth into a walrus-like creature.
Like fairies, imps retain their shallow and self-absorbed personalities, but they hate themselves greatly instead of being vain. Their vanity causes them misery. They become spiteful and jealous, wallowing in wretchedness.
Mortals who have not consumed magical milk will see imps as spiders or other insects. Seth's fairy turned into a hairy tarantula with striped legs and shiny black eyes. But in reality, imps have pointy teeth, a long, black tongue, brown, leathery skin, tattered ears, and shriveled, spindly limbs. Their eyes are black and they have a pair of slits for a nose. They can be either skinny or plump, depending on what they were as a fairy. They are usually taller than fairies.