A magical companion, usually an animal or spirit, bonded to a magic user and serving as partner, amplifier, or extension of their power.
A familiar is a supernatural entity (often taking animal form) that is magically bonded to a practitioner. The concept comes from European folklore, where witches were believed to have animal spirits that did their bidding. In modern fiction, familiars range from sentient talking cats to elemental spirits to daemon-like soul manifestations. What makes a familiar different from a regular pet is the magical bond: the familiar and its master are connected, and that connection has consequences.
Familiars are one of the best tools for externalizing your character's inner world. A familiar can voice doubts the character won't say aloud, reveal personality traits through their form or behavior, and create relationships that don't require other human characters. They're also great for pacing; a scene with a character alone in a room becomes a scene with dialogue when a familiar is present.
Daemons are the gold standard. Every person has one, they reflect your personality, and being separated from yours is the most visceral horror in the series.
Flips the trope by making the familiar (Bartimaeus) more compelling than his master, and by exploring the ethics of binding a sentient being to servitude.
Ged's otak is a small, quiet companion that grounds the character in warmth during his darkest moments.
Give the familiar a role in the story beyond looking cool. They should reveal character, create conflict, or serve a plot function. If you could remove them and nothing changes, they're decoration.
If your character has a bonded companion, they should be present and accounted for in every scene. Readers will notice when the raven on your hero's shoulder vanishes for fifty pages.
The bond should matter. If the familiar is hurt, the master should feel it. If the master is corrupted, the familiar should change. A bond with no stakes is just pet ownership.
Create a familiar for your protagonist. Decide what form it takes and why that form reflects (or contradicts) the character's personality. Write a scene where the familiar and the character disagree about a decision, and let the argument reveal something the character has been hiding from themselves.