Ryōhei Sudō, an 18-year-old third-year student at Suginami Second High School, stands at 178.3 cm with a lean 59 kg frame and blood type A. His youth was shattered by the loss of his father and brother in an accident, leaving his mother and childhood confidant Toujuurou Yagihashi as his sole pillars of support. Their subsequent murders at the hands of the enigmatic Koutei forged an unyielding determination within him to eradicate Koutei’s legacy, regardless of the cost.

Renowned for a strategic intellect that secured him the nation’s top academic rankings, Sudō wields the "Power of Assumption"—an analytical ability to simulate countless scenarios, extract hidden truths, and transmute threats into psychological endurance. This skill underpins his role as a master tactician, compensating for the intangible "Kakuhoken" privilege that offers no inherent advantages beyond his mental acuity.

Presumed dead after a brutal confrontation with Koutei’s enforcer Hisoka Midō, Sudō was interred by Soutarou Shinozaki following his last act of passing vital intelligence to Saneatsu Narita. His survival, enabled by the covert "Suspension Privilege" and Toujuurou’s posthumous findings, led to a clandestine recovery. Returning as the group’s linchpin strategist, he combines rapid physical recuperation with ironclad mental resilience to thwart psychological assaults and orchestrate counterplays against Koutei’s lethal schemes.

Formal in address—reserving first-name familiarity solely for Toujuurou during a pivotal courtroom encounter—Sudō shares a bond with his late friend defined by competitive trust, detailed in supplementary comics where their collaborative ingenuity unravels complex challenges. His foresight remains indispensable in outmaneuvering the survival game’s deadly majority rule, cementing his role as the architect of resistance against an omnipresent adversary.