Io Naruko is a second-year student at Binan High School, born on December 17th (Sagittarius) with blood type A. He actively pursues financial investment and management, generating significant income through stock trading even as a high school student. This financial acumen fuels his life motto: "Life is all about money!" Beef bowls are his favorite food.

He possesses pale green hair that reaches his collar, featuring long bangs swept across his forehead, and light grey eyes. Standing 178 cm tall, he ranks as the third tallest member of the Earth Defense Club. His standard uniform consists of a white shirt, black blazer, black pants, a red tie, and brown shoes. When transformed into his Battle Lover form, Roaring Prince Battle Lover Sulfur, he wears a prince-inspired outfit: white puffed sleeves with yellow cuffs, a yellow bow adorned with a heart-shaped jewel, white silk gloves, knee-length white shorts trimmed in yellow, and yellow boots.

As an Earth Defense Club member wielding the earth element represented by yellow, he combats entities spreading hatred and discord, frequently facing the Conquest Club. His transformation sequence and attacks utilize the "love stick" named Raging Gaia, culminating in his signature move, Sulfur Gaia.

His personality blends pragmatism and level-headedness, often prioritizing practical concerns like financial implications during crises. He shares a close friendship with fellow club member Ryuu Zaou, addressing each other informally as "Io" and "Ryuu." Their bond includes gestures suggesting deeper affection, such as Io arranging logistical support for Ryuu on Valentine's Day and showing visible pleasure when receiving chocolates from him.