He's motivated to help his friend Claudio get with Hero. He does this when the mask party is going on. He said to Claudio he promises that at Leonato's "revelling" that night he will disguise himself as Claudio, win Hero's heart, and make the proper arrangements with Leonato, so that "the conclusion is, she shall be thine." Then at the revelling Don Pedro is in disguise and starts talking to Hero but then he takes off the mask and surprises her and kisses her hand. Claudio is mad, but later he talks with her.
He is motivated to make his friends happy. In addition to Claudio and Hero, he also helps Beatrice and Benedick.
He wears a suit, he is dark skinned and is related to Don John the bastard. Don Pedro is the only one who doesn't end up with a wife this says that he is just looking for the right women. Don Pedro is mirthful through the play and can crack a joke anytime but he is not down on marriage as the others are. He notices beatrice and proposes to her but she just shrugs him off, but he is not sad. He stays the same and looks for a good husband for beatrice. Don Pedro say's " Lady Beatrice, i will get you one" when she says i may sit in a corner and cry for a husband.
Impact on Plot and Theme
Don Pedro impacts the the plot and theme because he is a main character. With out him the ending would be different because Claudio and Hero wouldn't get married and Beatrice and Benedick wouldn't either. He adds to the theme by getting his friends into relationships. He loves Beatrice but she rejected him. I would say he enjoys more of a lasting relationship because he asked Beatrice to marry him.