Genma Isurugi

is the principal of Saint Ishiyama Academy. ok he is old man with big muscle

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Saint Saint X'mas Arc
Late on Christmas Eve, Isurugi, disguised as Santa Claus, comes to the Oga residence so that he can fulfill the tradition of a "real" Santa Claus visiting after the Saint Saint X'mas competition. However, he is locked out of the household, though he states that everything has still ended well as he freezes in the snowing night. Isurugi ends up leaving a wrapped package of cheese near the front gate, which, due to the homophonous name of a present Hilda wanted to get for Baby Beel, is also of the Gorgonzola kind.

Relationships
It seems that he has some sort of affiliation with Aoi's grandfather and Saotome, as the three of them assaulted Behemoth together