Io is portrayed with pale green hair that reaches his collar, with long bangs parted to either side of his forehead. He has light gray eyes and is the third tallest among the members of the Earth Defense Club, giving him a presence that balances youthful sharpness with a composed look.

His Binan High School uniform consists of a white shirt tucked into long black trousers, a black blazer worn buttoned, and a red tie, paired with brown shoes. His neatly kept attire reflects his straightforward and practical personality.

When transformed into the Roaring Prince, Battle Lover Sulphur, Io wears a prince-inspired costume. His outfit includes a white jacket with puffed sleeves and yellow turned-up cuffs, extending into six long coat tails lined with yellow fabric. A yellow bow tied beneath his collar features a winged heart-shaped jewel at the center. A frilly white undershirt peeks out beneath the jacket, complemented by white silk gloves. He also wears white knee-length shorts trimmed in yellow and yellow boots, which are folded at the top to reveal a white inner lining. The costume highlights his association with the earth element, using warm yellow tones to emphasize his role as Battle Lover Sulphur.

Io is characterized as one of the more level-headed and pragmatic members of the Earth Defense Club, often providing rational observations during chaotic situations. This trait is evident from the first episode, where he quickly identifies the larger threat posed by a monster while others are distracted.

He is strongly motivated by his love of money and economics, frequently viewing situations through the lens of profit and value. His knowledge of commerce and financial matters sets him apart from his teammates, giving him a sense of worldly awareness unusual for a high school student. While this obsession with money can make him appear materialistic, it also highlights his practicality and calculated approach compared to the more carefree or impulsive personalities in the group.

Io Naruko is a second-year student at Binan High School and one of the five founding members of the Earth Defense Club. Unlike many of his peers, Io demonstrates an unusual maturity when it comes to financial matters, supporting himself through foreign exchange (FX) trading. His fixation on money and economics often informs both his outlook and decision-making, making him the most commercially minded member of the group.

Io’s involvement with the Earth Defense Club begins after the arrival of the mysterious pink wombat-like creature, who bestows magical powers upon him and his classmates. Chosen to represent the element of earth, Io transforms into Battle Lover Sulphur, adopting the title of “Roaring Prince.” Though not as outwardly expressive as some of his teammates, he contributes to the team with his analytical thinking and steady nature, providing balance to the group’s more impulsive personalities.

Ryuu Zaō
Io shares his closest bond with Ryuu Zaō, his classmate and fellow member of the Earth Defense Club. Their friendship is a central aspect of both their characters, and while it initially appeared strained, it eventually developed into one of the strongest connections within the group.

In their backstory, revealed in the second season, Io and Ryuu did not get along at first. Io disapproved of Ryuu’s focus on dating, dismissing it as “vulgar” and unprofitable, while Ryuu criticized Io’s fixation on money, remarking that valuing only wealth was shallow. Their tension began to dissolve during a visit to the Kurotama bathhouse, where an incident involving Yumoto and Gora led to a reconciliation. From that point forward, their relationship grew closer, and they often acted as a duo within the Defense Club.

Their bond is portrayed with humorous and occasionally romantic undertones. In Episode 6, Io only joined the battle after seeing Ryuu in danger, suggesting a protective streak. Later, in Episode 9, Ryuu gave Io chocolates for Valentine’s Day, which Io accepted with a blush despite his outward indifference. Their dynamic is frequently teased by other characters, such as En Yufuin, who once suggested they might harbor deeper feelings for each other. While both quickly denied it, their strong connection remains a recurring theme in the series.

The given name Io (硫黄) translates directly to “sulfur,” referencing the chemical element with atomic number 16, a yellow, combustible nonmetal. His elemental association as Battle Lover Sulphur reflects this naming choice.

His surname, Naruko (鳴子), is composed of the kanji naru (鳴), meaning “ringing” or “sounding,” and ko (子), meaning “child.” The name is also derived from Naruko Onsen, a well-known hot spring located in Miyagi Prefecture, in the Tōhoku region of Japan. Like the other members of the Earth Defense Club, his surname references a famous Japanese hot spring.