There is no opposite gender for the word "mediator." This is it is a gender-neutral noun that can be used by any gender.
Examples of antonyms for the noun 'mediator' are:agitatordisruptorprovocateurtroublemaker
Female=mediatrix
Female=mediatrix
Doctor is a neuter gender
Clot is not gender specific.
An emperor is the opposite in gender to an empress.
Opposite gender of Filipino: Filipina
The opposite gender of fraternity is sorority.
The opposite gender of giantess is giant.
Opposite gender to negro is-- NEGRESS
The opposite gender is girl.
The opposite of dairymaid is dairyman.