go-between vs treater

go-between and treater both are nouns.

Word	Noun	Adjective	Verb	Adverb
go-between	Yes	No	No	No
treater	Yes	No	No	No

As nouns, treater is a hypernym of go-between; that is, treater is a word with a broader meaning than go-between:

go-between: a negotiator who acts as a link between parties
treater: someone who negotiates (confers with others in order to reach a settlement)

Other hypernyms of go-between include negotiant, negotiator.

go-between (noun)	treater (noun)
a negotiator who acts as a link between parties	someone who negotiates (confers with others in order to reach a settlement)