Combat Agent Six, designated Combatant Number Six, serves as a high-ranking field operative for the globally dominant Kisaragi Corporation. He joined part-time at age 19 during his first high school year, accumulating three to four years of service to become one of Kisaragi's longest-tenured combat agents besides its founders. Life-saving reconstruction after shielding superiors from an attacker replaced biological components with cybernetics, fundamentally enhancing his physique.

Despite seniority, Six maintains low corporate rank by squandering performance-based "Evil Points" on snacks, pornography, and video games instead of upgrades. Consequently, his outdated cybernetics—while granting peak human physicality, three-minute space survival, night vision, underwater sight, extended breath-holding, nano-machine-based regeneration, disease immunity, and a neural translation chip—remain unoptimized.

His moral code blends overt pragmatism with concealed altruism. Missions employ underhanded tactics: sabotaging supply lines, hostage-taking, distraction exploitation, and sexual harassment to provoke emotional reactions. Standard gear includes a strength-amplifying first-gen power armor (with post-use paralysis), a pistol, combat knife, and his favored R-Buzzsaw—a high-frequency blade slicing hardened materials. Additional equipment is teleported via wristwatch using Evil Points earned through villainous acts. However, his evil remains superficial, limited to petty crimes like misconduct or minor sabotage; inherent decency prevents true atrocities like betrayal or abandoning allies. This, alongside loyalty and refusal to consume sentient beings, stalls his advancement.

Interpersonally, he shares operational rapport with assigned partner Alice Kisaragi, an android curbing his impulsiveness. He pursues romantic interest in Supreme Leader Astaroth, inspired by her smile during his recruitment. With fellow leader Lilith, he maintains a contentious yet familial bond over shared recreation. In his primary fantasy-world mission—scouting for invasion while embedded in medieval Europe's Kingdom of Grace—he interacts significantly with knight Snow (initially harassed, later a trusted comrade), chimera Rose (who sees him as a commander), and dark mage Grimm (accidental engagement pact turned mutual fallback marriage). His lewd conduct includes photographing demonic Heavenly King Heine for personal gratification.

Initially focused on Evil Point accumulation, repeated collaborations with locals and clashes with the Demon King's army gradually forge responsibility. Key shifts include fury at Grimm's temporary death and sacrificing his Evil Points—and chance to return to Earth—to secure a weapon defending the kingdom. This cements his evolution from self-serving opportunism to reluctant guardianship, anchoring him as the fantasy world's protector despite nominal allegiance to Kisaragi.