One of the original eight demons who closely supported the Fuujin oni, reincarnated in the body of a frog and thus took the shape of his host.

The first demon sent after Yaiba, he led a few skirmishes against Yaiba's party, but was eventually defeated and captured. Shortly after, the Snakeman demon appeared to both attack Yaiba and get rid of the Frogman due to his uselessness. Gerozaemon, with no place to return and a peaceful nature, was welcomed into Yaiba's group after the battle against the Snakeman.

A melee fighter, he favors the use of body-slam techniques reminiscent of sumo, but can also skillfully use axes and his own tongue as weapons when needed.