Familiars are magical beings of any shape or form or race that has entered a pact of service to a magic-user. Familiars can be of any magical race, from Demons to Magma Mice (humans are not included in this, no matter the form). When a magical beast enters this pact, they give over their body, mind, and soul to the service of their master, who may summon their aid or simply their companionship. Only the familiar's master can break the contract, and all familiars have total loyalty to their masters, serving their every whim.
If a familiar is killed, the master will suffer incredible pain, as a familiar is literally part of the magic-user's person. Should the magic-user die, then the familiar will die through a horrifying process of rapid aging and decay, until the familiar lies as dust and bones on the ground.
Familiars have many small details that incorporate themselves into the existence of Familiars. It is, for example, wrong it say Familiars have free will. They obey the will of their masters happily, and take great pains to ensure their master is both content and comfortable.
This means that the idea of hurting their master or themselves in any way is totally alien and deplorable to the Familiar. Familiars cannot commit suicide, unless directly told to do so by their masters.
Familiars have nothing but adoration and admiring for their masters, and will defend their masters to death's embrace. While a Familiar can be sarcastic in speech, they will still harbor this undying love.
Familiars can be temporarily freed of their service, for example a master could state "I free you for 1 week." In this case, if this is spoken on a Tuesday, the Familiar is free for 1 week and will then be bound again to the contract. Of course, the Familiar is still bound by the no self-harm rule.