Yamakoshi, also called "Yamakoshi the Cruel Fish" and "The Cruel Fish", is a antagonist and pet cruel fish of Lanny and a character in Pair of Kings. Yamakoshi, he doesn't like Brady and Boomer, he wants to get rid of them so Lanny can become the king.
In the season 1, Yamakoshi was trying to tell Lanny to do bad things by forcing Lanny to try and become king by replacing Brady and Boomer (not that Lanny minds), he's really grumpy and he doesn't like the kings Brady and Boomer, he wants just have a clean aquarium, but he uses the water for not forcing Lanny in the season 2.
Its later revealed in "The Evil King" he is really King Kalakai, one of the two first kings of Kinkow. The Evil King, Kalakai became obssesed with envy over his twin brother, turning him evil. He tried to kill his brother, but was defeated. He created the darkside of the island as his own dark kingdom. He was defeated however and turned into a fish. Lanny accidentally turned him back to human form by placing him upon the Evil Kings throne.
Now returned he was however limited in the fact he couldn't leave his castle or he would return to fish form. As such he had Lanny use dark magic to corrupt King Brady, to bring him the magical king's rings. With them he could regain his full powers and take the kingdom for himself. He almost succeeded, however Mason attacked him and this managed to distract him long enough, for Boomer to free Brady from the curse. Retaking their rings, Kalakai tried to destroy them, but turning the power of the rings against him, they managed to defeat Kalakai, knocking him apparantly to his death.
Unknown to anyone else, he was returned to fish form and went back to Lanny. However he was never seen again.