Poseidon

ポセイ・どん

A member of the merman race who appears to be a homage to the monster from The Creature from the Black Lagoon. He is much larger than humans and other mermen, with a size comparable to hills and buildings. He lost a duel to Riza. In the manga, he died after the duel, but in the anime, he is still alive and has fallen in love with Riza. He was unnamed in the manga but was named Poseidon in the anime.