Mensch (מענטש) a Yiddish word that means "a person of integrity." A mensch is someone who is responsible, has a sense of right and wrong and is the sort of person other people look up to. In English the word has come to mean "a good guy." Menschlichkeit (מענטשלעכקייט) is a related Yiddish word used to describe the collective qualities that make someone a mensch.
The opposite of a mensch is an unmensch, which means an evil or cruel person.