harmonizer vs go-between

harmonizer and go-between both are nouns.

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

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

harmonizer: a mediator who brings one thing into harmonious agreement with another
go-between: a negotiator who acts as a link between parties

Other hypernyms of harmonizer include intercessor, intermediary, intermediator, mediator.

harmonizer (noun)	go-between (noun)
a mediator who brings one thing into harmonious agreement with another	a negotiator who acts as a link between parties
a musician who sings or plays in harmony