fill-in vs compeer

fill-in and compeer both are nouns.

Word	Noun	Adjective	Verb	Adverb
fill-in	Yes	No	No	No
compeer	Yes	No	No	No

As nouns, compeer is a hypernym of fill-in; that is, compeer is a word with a broader meaning than fill-in:

fill-in: someone who takes the place of another (as when things get dangerous or difficult)
compeer: a person who is of equal standing with another in a group

Other hypernyms of fill-in include equal, match, peer.

fill-in (noun)	compeer (noun)
someone who takes the place of another (as when things get dangerous or difficult)	a person who is of equal standing with another in a group