Hinomaru Ushio, a two-time elementary school sumo champion hailed as the "National Treasure Onimaru Kunitsuna," saw his junior high career stall due to his short stature (152 cm pre-time skip) and three consecutive losses, leading him to quit competitive sumo for three years. He revived his ambitions upon entering Oodachi High School, joining the sumo club to pursue becoming a yokozuna despite being below the professional height requirement of 167 cm. Orphaned, he was raised by his grandparents, with his mother's death significantly impacting his emotional development.

Physically, he possesses reddish-black hair depicted as flame-like, amber eyes, and a scar on his lower left cheek, with numerous other scars covering his muscular body from training. Pre-time skip, he stood 152 cm tall and weighed 79 kg, typically wearing his school gakuran or draping a jacket over his mawashi like a cape. Post-time skip, his height reached 157 cm and weight 93 kg. He adopted a traditional topknot and wore a black-and-red flame-motif yukata bearing his shikona ("Onimaru Kunitsuna"), though he preferred casual T-shirts and pants at home.

Personality-wise, he exhibits a friendly, easygoing nature in daily life but becomes intensely focused during matches, actively embracing crowd energy unlike peers who block it out. Deep-seated trauma from his mother's death fuels intense anger at jokes about death or threats of abandonment. He expresses discomfort with romantic attention, calling himself a "sumo freak" incapable of emotional reciprocity, avoids "dirty talk," and apologizes to love interest Reina Gojo for perceived "uncool" behavior. His defining flaw is a reckless obsession with victory, disregarding personal safety—a trait rivals label a "death wish." This stems from extreme self-reliance developed in childhood due to inadequate care, making him reluctant to depend on others. He is notably old-fashioned and technologically averse, favoring pay phones over mobiles and running long distances to distribute physical flyers rather than using digital versions.

In combat, he employs "yokozuna-style" sumo, leveraging his low center of gravity for head-on attacks. His signature technique is the "Demon Wheel" (right underarm throw), and he integrates adapted karate strikes to maintain distance. Despite his size, he demonstrates "demon-like" strength and endurance, winning individual bouts through improvisation, such as defeating Chihiro Kunisaki with a move derived from Kunisaki's suplexes. However, his techniques proved susceptible to analytical counters due to lacking formal scouting exposure in junior high. Post-time skip, he refined his approach, incorporating feints and varied setups like the "Reverse Hundred Demon Throw" to enhance unpredictability.

His development spans high school to professional sumo. After leading Oodachi High to a national team championship and placing third individually, he bypassed the height requirement via sandanme tsukedashi, joining Shibakiyama Stable. Early professional success was interrupted by a severe right-arm injury, sidelining him for two years. Returning to the top makuuchi division, he confronted dominant yokozuna Jinō, whose strength motivated Japanese wrestlers to train collectively. During the September tournament, he overcame losses by relying on former teammates for strategy and publicly proposed to Reina Gojo mid-tournament. He ultimately defeated Jinō in a playoff to win the championship, marrying Reina six months later while continuing his professional career.