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Claudio is a starry-eyed young kid: good but totally pliable. Seriously, he acts like he's about twelve.
He’s a friend of Benedick and would-be groom (and then actual groom), of Hero. Our first introduction to Claudio is from a messenger: we learn that he’s fought well in battle, but on hearing of how highly he is praised, Claudio weeps.
Leonato brushes Claudio’s tears off as a good trait, but it’s an introduction to the immaturity that will characterize Claudio for the rest of the play. Claudio’s actions throughout the play reveal that he has quite a bit of growing up to do—he falls in love with Hero over the course of one silent meeting, can’t even court her on his own, and then resigns her entirely at the hint that she might like Don Pedro better. Claudio isn’t only immature in his dealings with his own life. When he deals with Benedick’s love for Beatrice, he’s playful in his teasing, but he can’t take Benedick seriously, even when Benedick is actually being serious. He lacks insight, and the intuition to critically assess the situations he’s in, but he’s always willing to be furiously passionate.
His passionate feelings, and the enthusiasm with which he gives himself up to his emotions, are as marked as his immaturity. When Claudio loves Hero, he loves her to distraction. When he hates her, he hates her with fury:
CLAUDIO If I see anything tonight why I should not marry her, tomorrow in the congregation, where I should wed, there will I shame her. (3.2.116-118)
Yeah, he hates her so much that he needs to destroy her... he's not content simply not marrying her. Jerk.
And it continues. When he teases Benedick, he teases mercilessly. When he realizes he has wronged Leonato, he’ll do anything to win Leonato’s forgiveness. He mourns Hero like one who will never love again, and then happily will marry whoever Leonato puts in front of him the next morning. Only when he sees Hero again does he simply say "Another Hero!" with no more gushing.
This is suspect—perhaps this means he’s learned his lesson about being too effusive, and he’s now cowed by his modesty. But actually, no. He speaks again, and his last words in the play are to tease Benedick about inevitable disloyalty in Benedick’s marriage. Sometimes a character’s last lines can be as telling as their first lines—through Claudio’s farewell in the play, we see that he hasn’t learned all that much. He’s still prone to youthful idiocy, and it makes us wary that perhaps he’ll be as prone to youthful rashness as well.
Well, wait a minute though, you’re saying, that can’t possibly be all there is to Claudio, who is our kind of would-be hero of the play. Surely, we’ve left something out?!
Oh yes, you’re right: he’s gullible and easily manipulated. He also has no capacity for modesty or real apology. We don’t mean to be too harsh on the guy, but Claudio doesn’t do much to endear himself to us in this play. Perhaps his worst failing is shown after Don John’s villainy is resolved. In front of Leonato, after Borachio’s repentance, Claudio has a chance to admit that he realizes how easily he’s manipulated, and apologize for his lack of critical thinking. Instead, he struggles to cover his butt. He tells Leonato he’s really sorry, but his only sin was in mistaking. (Imagine standing in front of a father whose innocent daughter you’ve just defamed and killed, and saying, "Oops! Sorry! Well, it wasn’t really my fault." Weak sauce.)
By the end of the play, he’s still a merry, foolish boy, and while he’s apologized, he’s never really come to terms with his own personal failings. His willingness to fall under a passion leaves him open to be easily manipulated and deceived by the merest suggestion. Unfortunately, when the play ends, we have no assurance that he won’t fall under manipulation again later. Overall, Claudio is characterized by a penchant for believing the story of the day with passion, and acting on that belief as though it were gospel.