playmate vs companion

playmate and companion both are nouns.

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

As nouns, companion is a hypernym of playmate; that is, companion is a word with a broader meaning than playmate:

Other hypernyms of playmate include associate, comrade, familiar, fellow.

playmate (noun)	companion (noun)
a companion at play	a friend who is frequently in the company of another
	one paid to accompany or assist or live with another
	a traveler who accompanies you

playmate (verb)	companion (verb)
	be a companion to somebody