Hideo was hired as a teacher just because he was a relative to the chairperson of the school. He was a criminal who was taking his students out of school for relationship money.

(Source: TYPE-MOON Wiki)

January, 1999: Apprentice mage Azaka Kokutou, Mikiya's younger sister, has been ordered by her mentor, Touko Aozaki, to investigate a certain incident in which fairies steal the memories of students at Azaka's school, Reien Academy. Azaka launches an investigation with the help of Shiki.
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(Source: Aniplex USA)

Hideo Hayama