There are too many senses in which the noun "familiar" may be used.

In the play Bell, Book and Candle, it describes Gillian's cat Pyewacket, an embodied spirit with supernatural powers who attends, assists and protects a witch or wizard.

In the television series Cheers it describes the characters who gathered routinely at the bar. The American synonym would be "regulars."

It may describe a person who is routinely in the company of another, a companion or associate.

In more antique use, it describes a person who performs domestic services in the home of a high official, especially a religious official, such as a pope or a bishop.

In a narrow and specialized use, it refers to a person who both lives and works inside a Christian monastery but has yet to take any vow.

And it may describe a person who is learned or well-versed or highly acquianted with something (usually followed by a prepositional phrase, e.g. "familiars of the law").

Are there others?