Yuuma Chiaki, widely recognized as Thor, rose to prominence as the Seventh Fist within the delinquent powerhouse Ragnarök. He joined the organization driven by a singular ambition: to propel his unique combat-sumo fighting style into the mainstream and secure the influence necessary for its establishment. His path prior to Ragnarök remains shrouded, marked only by numerous street fights where he honed his developing techniques. Within Ragnarök, Thor forged a profound bond with Siegfried, the Fifth Fist, considering him his sole true comrade. He operated the underground fighting arena known as the Sumo Stable, utilizing it to refine his skills and command his followers.

Thor possesses a towering, rotund frame blending substantial muscle with fat, a physical composition integral to his defensive prowess. His appearance features short black hair, long sideburns, and a characteristic sleeveless kimono, often worn barefoot. After aligning with the Shinpaku Alliance, he accepted steel arm guards forged by Shigure Kōsaka, designed to resist weapons without compromising his sumo techniques.

His personality intertwines deep-seated honor, sharp perceptiveness, and unwavering loyalty. Despite his Ragnarök affiliation, he adhered rigidly to sumo's rules and principles, readily engaging opponents in reasoned dialogue and honoring pacts. Following defeat by Kenichi Shirahama, Thor upheld their duel's terms by departing Ragnarök, an act that invited severe retribution. He subsequently allied with the Shinpaku Alliance, embracing its members as genuine comrades. Thor displays fervent support for fellow sumo wrestlers during tournaments. He harbors a fear of heights and intense amusement park rides, though he conquers the former when allies require aid.

Thor's signature combat-sumo is a self-crafted hybrid martial art fusing traditional sumo with street-fighting pragmatism, incorporating strikes, throws, and takedowns. Remarkably, he attained disciple-level prowess entirely without formal instruction, a feat acknowledged as exceptional by peers like Ikki Takeda. His immense strength allows feats like lifting boulders twice his size, while his unique muscle-fat composition grants enhanced endurance, absorbing significant damage—a resilience fortified through Valkyrie training involving repeated abdominal strikes. After Ragnarök's dissolution, his speed increased markedly, enabling him to dodge weapons despite his bulk. Key techniques include the concrete-shattering stomp "Hurricane Wave Times 4," the palm strike "Oniharite" enhanced by a wrist twist, the throwing maneuver "Sabaori," and the joined-fists smash "Thor's Hammer."

His development intensified under the mentorship of Danki Kugatachi, Freya's grandfather and a master of the staff. Danki's tutelage taught Thor weapon evasion, refined his movement efficiency, and enabled him to adapt staff-fighting principles to sumo, significantly bolstering his versatility. This training solidified his resolve to perfect combat sumo. Throughout the YOMI conflict, he engaged in pivotal battles, including the D of D Tournament where he collaborated with Shinpaku members to overcome adversaries like the Black Force Team. He sustained grave injuries shielding Miu Fūrinji from Shō Kanō but later contributed to clashes against YOMI fighters such as Rimi and Lugh. His growth culminated in accepting Shigure's arm guards, symbolizing his full integration into Shinpaku and his enduring dedication to martial refinement.