Issei is the Student Body President of Shirō's school, and another close friend of Shirō. Issei often asks Shirō to fix broken equipment for the Student Body to try and reduce save money on the school's bill. Issei and Shirō often have lunch together in the Student Body's office where they talk about what ever comes to mind. His father is the head priest of Ryūdōji Temple, and his brother was a classmate of Taiga and Neko-san. He dislikes Rin and seems to have some sort of fear of females, although he seems to be unusually at ease around Saber.

Spoiler

Issei unknowingly tells Shirō that Caster is at the Temple.

(Wikipedia)