Io Naruko, a second-year Binan High School student and Earth Defense Club member, navigates life through a pragmatic lens shaped by his conviction that financial success underpins existence. This philosophy fuels his lucrative ventures in investment and money management, uncommon pursuits for a teenager. His closest bond lies with Ryuu Zaō, a fellow second-year; their friendship blends steadfast loyalty with flickers of romance, evident in exchanged Valentine’s chocolates and Io’s flustered reactions to Ryuu’s gestures. As Battle Lover Sulphur, Io channels the earth element, his Roaring Prince persona clad in frilled yellow regalia. He wields the Raging Gaia weapon to unleash the Sulfur Gaia strike during battles. Standing at 178cm with pale green hair and ash-gray eyes, he ranks as the club’s third-tallest member. Io’s analytical mind cuts through chaos, prioritizing monster threats over distractions during crises. His mercantile instincts and strategic planning set him apart within the group, showcasing a maturity beyond his peers. While Ryuu remains his primary emotional anchor, he coordinates seamlessly with other defenders against foes like the Conquest Club and evolving antagonists. Consistent across TV seasons, OVAs, and the *Eternal Love!* film, Io serves as the Defense Club’s unwavering pillar, rejoining allies in 2025 to confront fresh threats. Though his backstory and future beyond school remain uncharted, his character retains defined traits: a beef bowl enthusiast whose composed exterior occasionally cracks to reveal vulnerability, particularly in Ryuu’s presence.