Io Naruko 鳴子硫黄

Io Naruko is a 2nd year at Binan High School. Despite being in high school, he makes good money trading on the FX stock exchange. His life motto is "Life is all about money!". He is good friends with Zaō, who along with being a member of the Battle Lovers, is in his graduating class. They call each other "Ryuu" and "Io". It is suggested that they have romantic feelings for each other or a deep friendship that has some romantic dimensions.

As a Battle Lover, his given element is Earth, and he transforms into "Roaring Prince, Battle Lover Sulphur" (Todoroki Ouji, Battle Lover Sulphur).