familiar vs playmate

familiar and playmate both are nouns.

Word	Noun	Adjective	Verb	Adverb
familiar	Yes	Yes	No	No
playmate	Yes	No	No	No

As nouns, playmate is a hyponym of familiar; that is, playmate is a word with a more specific, narrower meaning than familiar:

Other hyponyms of familiar include date, escort, playfellow, tovarich, tovarisch.

familiar (noun)	playmate (noun)
a spirit (usually in animal form) that acts as an assistant to a witch or wizard	a companion at play
a person attached to the household of a high official (as a pope or bishop) who renders service in return for support
a friend who is frequently in the company of another

familiar (adjective)	playmate (adjective)
well known or easily recognized
within normal everyday experience; common and ordinary; not strange
having mutual interests or affections; of established friendship
(usually followed by `with') well informed about or knowing thoroughly