Aoi, a normal school boy, is cursed by a wizard named Makaeru from the Land of Frogs, who tells him that if he wants to remove the curse he is to find Mimori, who is later discovered to be the sister of Makaeru. The curse causes Aoi to transform into a frog whenever he is wet, changing back into a human when he dries off. Makaeru then casts Aoi into the Land of Frogs, where he meets Mimori, the princess of the land. Aoi asks if she knew about the curse. However, Mimori remembers her brother's saying as he handed her the Book of Magic before he disappeared: "This book enables the frog people like us to use magic. If you find that there are some pages missing, then it is your duty to find it."
Aoi then realizes that his quest is to find the missing pages of the Book of Magic, hoping that the pages will contain the cure for the curse.
- Princess Mimori
Age: 14 Mimori is the main female character in the series. She is also the princess of the frog clan that is always happy and cheerful. It is known that Mimori has feelings for Aoi but has not realized it to her denseness.
A normal boy from the real world. He has the ability to change to a frog when he gets wet by the curse of Mimori's older brother, Makaeru. Also, Aoi has feelings for Mimori.
The land of Frogs and the land of Snakes (the two different species) had been in war, as explained in the first section of episode 1. Mimori's parents had been absorbed into a tunnel, while engaged in the battle, and left only Makaeru and Mimori to govern the kingdom.
There were a minor number of survivors from the snake clan, but Mimori soon discovers a rival: the princess from the Land of Snakes, who had survived the war, and whose parents were also absorbed. This princess begins an interest to Aoi, and is jealous of Mimori, at which she finds a sailor (whose true identity is a woman) and falls in love with her.
The sound "kero" in Japanese (as in the title) is the "ribbit" or croaking of frogs, which, in the series, is used frequently by the frogs. It is also by Mimori, as she is the princess of the kind.