Fairies are linked to living things in nature - plants, animals, fungi, etc. They can appear nearby to any occurrence of the thing their magic is linked to.
Multiple fairies can be linked to different forms of the same thing, for example an arctic fox in its summer and winter coat. If the linked thing is weak, then while not in season they might hibernate or turn into an inanimate object that can be kept somewhere safe by its brethren. If the linked thing grows in power, an extra fairy might be born.
Fairies are always tiny, but they can swarm and dance in large numbers.
Other examples: Dandelion, Sparrow, Mushroom, Moth, Maple tree, Maple tree in autumn, Apple tree, Apple
Imps (or sprites, goblins, demons…) are elementals, linked to weather phenomena, geological formations, or other odd things, like the reflection of the moon in a certain lake.
An imp can change form along with its associated source of magic. For example, a lake sprite might have a friendly look from the lake when it’s warm, and an ice-queen look from when the lake freezes over. Their personality would change too.
A really strong imp can grow as large as a house or a mountain, although they become slow and sleepy along with it, and they blend into the landscape so well that you’ll probably never see one. An imp with multiple forms could also choose to split into a bunch of smaller imps, one for each form; these can rejoin later to share their emotions and memories.
Other examples: Frozen puddle, Fool’s gold, Rainbow, Mountain, Sinkhole, Forest fire, Lightning bolt, Altocumulus
Fairies vs Imps
The major difference between fairies and imps is that fairies linked to the same living thing are distinct people, while imps linked to the same natural phenomenon are merely different aspects of the same person.
Fairies habitually band together; they have a complex social structure involving a monarch, nobility, ministries, and various social customs, particularly parties and picnics. One wonders what all the point of this is when most fairies will never meet a government official and could easily run and hide from them rather than complying with such nonsense. However, fairies are social creatures and in fact their favourite methods of resolving conflicts are magical battles and public duels.
By contrast, imps are self-sufficient and may spend their entire existence isolated from any other living being. They can be scary and anti-social, or cheerful but difficult to understand. Imps ignore each other unless they want something. However, the cooperation between imps linked to the same natural phenomenon is flawless. In fact, they sometimes make fun of fairies for their disorganisation and bickering. However, imps' favourite method of resolving conflicts is also magical battles.
Fairies dress cutely, in white, pastel or natural colours. Imps prefer bold clothes in monochrome and dark colours. Both typically dress fancy, what might look to us like street, lolita or ouji fashion - after all, the clothes are maintained by magic, so the number of buttons and laces on one’s dress is of no concern.
Both fairies and imps can have all sorts of cute fantasy features such as animal ears, wings, fur, horns, tails and unusual colouring.