He is motivated to marry Hero because in the quote it says "I am returned and that war thoughts have left their places vacant, in their rooms come thronging soft and delicate desires, all prompting me how fair young Hero is". He loved her before he left to war and now that he's back he want to marry Hero really bad. Then his motivation changes when he "thinks" he saw Hero having sex with Borachio. He changes his motivation and calls Hero a slut and a whore. Then he realizes he did bad and Borachio comes clean and then Claudio marries Hero for reals this time.
He wear a military suit from and he is a soldier. He looks about 21 years old. People say that he is honorable, brave and promising. Leonato say's that Don Pedro honors Claudio. The messenger even say's " he hath borne himself beyond the promise of his age, doing in the figure of a lamb the feats of a lion."
Claudio sees "Hero" having sex with a guy(Borachio) so he starts assuming that Hero is cheating on him and that she's a slut. So he talks trash about Hero when they are about to get married, and throws her to the floor. We can see that his anger gets the best of him and he doesn't talk things out and doesn't think things through.
Impact on plot and theme
He does impact the plot because at first he is all lovey dovey and then he see's something that isn't true and he insults Hero. Then he believes another lie that is Hero "dying", he starts to feel really bad and then say's a few word at her funeral. Those words were that she wasn't a slut and all that stuff. He starts to regret insulting Hero and then he see's hero that she isn't really dead and hugs her and marries her for reals this time. Claudio loves Hero and the type of love they have is pragma.