The Frog King is an otherworldly entity ruling Frog Land, a realm accessed via an amusement park's Frog Castle. He challenges those who defeat the giant snake with a special sword from the castle, traditionally pledging to grant any wish to the victor. As guardian of this domain, he imposes a critical condition on entrants: they must open the Gate of Hope before a child emerges from a visible batch of roe. Failure transforms the individual into a frog.

His actions directly shape the central conflict for Lily after her victory over the snake, initiating the urgent trial tied to the hatching roe. Under his watch, Knodo abruptly appears claiming to be Lily's little brother, though the Frog King does not intervene in their ensuing chase.