Genma Isurugi

is the principal of Saint Ishiyama Academy.

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.

Powers & Abilities
Hand-to-Hand Combatant: Genma seems to have some skills in unarmed combat where it centers around powerful palm thrusts when he attacked Behemoth with Zenjuro Saotome and Ittosai Kunieda. Though as he is yet to be seen heavily engaged in combat it's unknown how high his overall skills are.

Muscle Enlargement: Genma possesses the special ability to enlarge his body's muscles at will in order for him possess great physical abilities in spite of his advanced age. It's unknown how he attain this ability but it seems to involve recalling the strength he had in his youth that brings forth this transformation.
 * Enhanced Strength: While in this state Genma is strong enough to physically attack his opponents from a far distance where his palm thrusts carries enough air pressure behind it to send Behemoth into the roof of the gymnasium while creating a large dent on the roof at the same time. Though despite the power behind this attack, Behemoth was able to withstand it with little damage when defending himself.

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