companion vs fellow

companion and fellow both are nouns.

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

As nouns, companion and fellow are synonyms defined as:

companion and fellow: a friend who is frequently in the company of another

Other synonyms of companion include associate, comrade, familiar.

companion (noun)	fellow (noun)
a friend who is frequently in the company of another	a boy or man
one paid to accompany or assist or live with another	a man who is the lover of a man or woman
a traveler who accompanies you	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

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