Claudio has been in the army for a while and has just now returned with the others. He has known of this girl Hero, but when he sees her for the first time from returning he begins to really like her. He talks about how he wants to be with her and that it might be too sudden. Hero has become a major motivation for Claudio and he now plans to marry her. "I would scarce trust myself, though I had sworn the contrary, if Hero would be my wife. (191)" He has become so in love with her that when he sees a girl who, looks like Hero, making out with another guy he calls the entire thing off, gets very upset and publicly shames her. He then realizes the girl wasn't actually her and now he has to marry girl who he knows as her cousin, but he only does because of how much he loved her.
Leonato: "A victory is twice itself when the achiever brings home full numbers. I find here that Don Pedro hath bestowed much honor on a young Florentine called Claudio."
Messenger: "...he hath borne himself beyond the promise of his age, doing in the figure of a lamb the feats of a lion."
Leonato and the Messanger are talking about Claudio and what they think about him. Leonato said that Don Pedro, the prince, has bestwed much honor on him, showing that he has faith and respects Claudio. The Messanger says he is good looking with a soft face, yet when he fights in war, he is like the mighty lion. Most people in this town really respect Claudio therefore when he publicly shames Hero, everyone believes it so easily.
Claudio: "...she knows the heat of a luxurious bed. Her blush is guiltiness, not modesty."
This is when Claudio is outing Hero for what he saw the night before. The entire town now thinks she is a harlot and no one went against because of how well he is respected. This shows how genuine he must be all the time if no one, especially the dad, goes against him to tell him that he is wrong about Hero.
Impact on Plot and Theme
One theme shown throughout the story was the importance of honor. The aborted wedding ceremony, where Claudio rejects Hero, accusing her of infidelity and publicly shaming her in front of her father, shows much honor towards Claudio. Hero's honor was built on her virginity and chaste behavior. When Claudio said she had lost it, she now would lose her social status where she could never recover it. The plot changes when we see how respected Claudio is when no one at the wedding doubts him for what he did. Also, it changes when we see how Hero's life is ruined by a person who Claudio thought was Hero.