Yamato Takahashi is a hundred precent convined that he is the main character of his life. Everyone, including his own sister, is a literal side character in his life and, as a result, he has the confidence of a narcissist and the inability to listen to reason.
It also seems that Yamato believes himself to be in a harem with a total of 12 other females, including his sister, his childhood friend, two subsitute faculty members, and several other classmates. Yamato, despite the unlikeliness of it all, is also thoroughly convinced that one of the girls in his 'harem' is a yandere.
Kaede Takahashi just wishes that her brother would shut up before she has an aneurysm.
