fellow vs playmate

fellow and playmate both are nouns.

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

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

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

fellow (noun)	playmate (noun)
a boy or man	a companion at play
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