"He was training a lower moon named Rui, although Kaigaku was a bad trainer he tried to teach Rui but failed, it seemed Kaigaku was a bad teacher, but also impatient..."

"Kaigaku had quit training Rui and started training his thunder breathing in hopes of becoming strong someday so when he finally faces the demon slayer who used thunder breathing, Zenitsu Agatsuma, he would kill Zenitsu with ease."

"Kaigaku was known as the demon who used thunder breathing."