playmate vs fellow

playmate and fellow both are nouns.

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

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

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

playmate (noun)	fellow (noun)
a companion at play	a boy or man
	a man who is the lover of a man or woman
	a friend who is frequently in the company of another
	a person who is member of one's class or profession
	an informal form of address for a man
	a member of a learned society
	one of a pair