Who is Mr. Kirwin and how does he treat Victor?

He is the magistrate who accuses Victor of Henry's murder. Since Victor knows he is not guilty, but he refuses to tell the truth about the monster, he is very conflicted on what to tell Mr. Kirwin next.

Mr. Kirwin is a magistrate.

"Ay, sir, free enough for honest folks. Mr. Kirwin is a magistrate, and you are to give an account of the death of a gentleman who was found murdered here last night."

Mr.Kirwin treated Victor with calm benevolence and firmness.

I was soon introduced into the presence of the magistrate, an old benevolent man with calm and mild manners. He looked upon me, however, with some degree of severity, and then, turning towards my conductors, he asked who appeared as witnesses on this occasion.