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The "juvenile delinquent" of the class, Katsuya Kimura (Kimura Katsuya 木村克也) acts much worse than he really is towards Nūbē and his other classmates. At the start of the series, Katsuya is consistently portrayed as a loner, either due to being misunderstood by his classmates, his own cowardice or from his delinquent behaviors such as smoking or stealing charity for a good cause. But as the series goes on, he slowly comes to grips with himself and joins in with Nūbē and his friends where he is one of the main members of the class. Katsuya is a huge pervert towards sexy females, an attribute that gets him into a huge idolation of Miki. (though she sees treats him like her dog) He also finds himself consistently with the worst grades in class alongside both Hiroshi and Makoto. But while he is a coward and a bad seed, he is amazingly also a loving, caring and protective big brother towards his little sister, Manami.

Source - Wikipedia