Keisuke is Miaka's older brother. He's very protective of Miaka and always thinks what's best for her. At first, when he heard the story about The Universe Of Four Gods from Miaka, he had problems believing her but to make sure, he told her not to go near the book. In spite of that, Miaka went and he went after her. It was not until when he saw her disappear into the book, did he believe the story and throughout a large part of the anime, he is the one who reads the book and at the same time tries to figure out how to help her return to the real world.

(Source: AnimeNfo)