Familiar

Familiars are magical creatures that act as companions for witches and spellcasters in The Sims 2: Apartment Life (when The Sims 2: Pets is also installed) and The Sims 4: Realm of Magic. Witches in The Sims 3: Supernatural get a moodlet that boosts spellcasting if they own a cat or a small pet, but these pets are completely normal otherwise.

The Sims 2: Apartment Life
If players also have Pets installed, witches can summon their own cat familiar called a spectral cat.

The Sims 4: Realm of Magic
In Realm of Magic, spellcasters can acquire magical familiars that can aid in spellcasting. If players own Cats &amp; Dogs, their pets can be bound as familiars as well. Spellcaster children can also own one, though they are unable to do magic until they grow up.

Interactions
Familiars are extremely helpful to spellcasters. They have magical energy reserves, aid spellcasters in magic duels, and can even protect them from death. Familiars must rest to regain their power after saving their owner's life. Cats and dogs (and by extension, raccoons and foxes) can be sent to forage for magical items.

Trivia

 * The fairy's appearance and one of its randomized names, "Hey", is a possible reference to Navi from The Legend of Zelda, a small glowing fairy who is known for saying "Hey! Listen!" to get the player's attention.