In the last episode, Haruma Yamazaki finally allowed Yamada-kun to continue his plan because he wanted to know why he became the student council president. Nevertheless, after the wish was granted, the reason why he became the president is still not clear to me - did I miss anything, or is the reason still unrevealed at this point?
Heads up. Spoiler from the manga.
Haruma Yamazaki became the president of the student council to save the Supernatural Studies Club from being abolished. Because only he and Leona Miyamura are the members of the club.
Source: Chapter 84