The utilitarist is that who adheres to utilitarism (the theory that cites the best thing to do is to sacrifice
“the happiness of one to the advantage of the majority”).
Everyone is an utilitarist in some way. I am being utilitarist now by writing this text in the example box to exemplify it. In this (example) case, is more of a (dub) case of “hedonist utilitarianism”.