Isabella is a young woman with a dream – she can't wait to become a nun, but her plans are thwarted when her brother is sentenced to death for the crime of fornication. Isabella is forced to leave the convent to seek out Angelo, who propositions her – if she has sex with him, he'll pardon her brother.
If we want to be sympathetic, we can also say the Duke is a protagonist in this play. All he wants is for his city to be free of crime and hypocrisy when Angelo, the guy who is supposed to be cleaning up Vienna, gets in his way. (See below.)