Familiars

A Familiar is a very particular type of Divine Vassal - a type of quasi-divine creature that originates on a realm ruled by a deity - that is assigned to a mortal master to aid and guide.

Appearance

As Familiars are divine vassals they can appear to be any insect, bird, reptile, mammal, or amphibian form native to their home plane. Although familiars may be assigned a different form when sent to the Prime Material Plane, all familiars revert to their "true" form when on their home plane.

Hence, on its home plane a familiar might appear to be a crow-shaped being formed of magma, but when sent to the Prime Material Plane they have the form of standard hummingbird.

Humanoid Familiars

Some familiars also retain the ability to return to their "true" form when on the Prime Material Plane. Notable familiars of this kind are Imps, Quasits, Baltir and the like, who all have the form of a small humanoid.

Purpose

All familiars have a specific purpose, which is to provide aid and advice to the servant of the Deity to which they are assigned. Without a specific mortal master, these creatures are "merely" proxies. And without being in service to a divine power they are just divine vassals.