Kotaru is the younger brother of Unchiko and Sei, and the youngest member of the Kaoru family. He lives Yokohama and he attends the Second Year of Middle Schools at the Nizi Nanatsu (The Seven Rainbrows). He doesn't like his sister, who he considers just a fat stupid woman. And he doesn't like his brother neither, because he's gay. He's very good at school and he gets the best grades very easily. His naughtiest classmates said that he's currently selling his body to the female-teachers in order to taint them to put him high grades, and so they call him "gigolò". His best friend at school is Daichi Kawai, the younger brother of Hachi Kawai, his sister's sweetie. Often they hang around and Kotaru complain with him about the ashaming and silly situations he've at his home. Natsumi Buta, twho's licterally drooling over him, always stalk him to ask him for a date, but since she's butt-ugly, Kotaru always refuses. He's perhaps the most normal in his family, apart from his hysterical tendencies, in fact he can easily lose his temper. Apart from this, he's very, very mature and he always do house duties of his brother and sister. He's also kind of generous but he doesn't like to show it up. He made up a kendou club in his school and he's quite good at practicing that. In fact he always train himself at home picking up his sister with his wooden sword, when she gets dumb (his sister, not his wooden sword -be careful). Since the three alien brothers always mess him up with their strange attacks, he asked Nyako to teach him the most easy technique he did know. Nyako did it, but he asked Kotaru to make his homeworks, since he cannot ask for a kiss.