harmoniser vs mediator

harmoniser and mediator both are nouns.

Word	Noun	Adjective	Verb	Adverb
harmoniser	Yes	No	No	No
mediator	Yes	No	No	No

As nouns, mediator is a hypernym of harmoniser; that is, mediator is a word with a broader meaning than harmoniser:

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

Other hypernyms of harmoniser include go-between, intercessor, intermediary, intermediator.

harmoniser (noun)	mediator (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