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 spell casting. If players own Cats &amp; Dogs, they can make their pets familiars as well. Spellcaster children can also own one.

Spellcaster help
Familiars are extremely helpful to spellcasters, they can reserve magical energies and can even help defend them from death. Familiars will get tired after defending a spellcaster from death and so must regain their energy.

Trivia

 * Hey is a reference to Legend of Zelda's Navi who was a ball of light with wings and frequently yelled "Hey!"
 * Voidy is a reference to the Veild Voidcritter featured in The Sims 4: Kids Room Stuff.