Michizō Tachihara first appears as a hot-tempered, impulsive member of the Port Mafia's Black Lizard battalion, marked by rust-red dyed hair, a nose bandage, and a military-style jacket over a white V-neck shirt. His outward demeanor radiates arrogance, a thirst for combat, and a readiness for violence, frequently clashing with colleagues like Gin Akutagawa and snapping at any perceived underestimation. This aggressive persona deliberately conceals his true allegiance as the fifth member of the government's elite Hunting Dogs unit, infiltrating the Port Mafia after the previous operative, Ango Sakaguchi, departed.

A history of familial resentment and crushing inferiority shaped his path. Tachihara chafed under constant comparisons to his older brother, deemed "perfect" by their family, pushing him toward delinquency and crime to forge a separate identity. Captured by the Hunting Dogs during a safe-cracking job, they coerced him into their ranks. The death of his brother—a soldier linked to Ōgai Mori and Akiko Yosano—deepened his sense of inadequacy, with his family blaming him for surviving. This trauma propelled his infiltration mission: to confront Yosano, whom he held responsible for his brother's death, and escape his brother's shadow by becoming "neither like him nor his opposite."

Throughout his undercover tenure, Tachihara meticulously guarded his true loyalty and the extent of his ability, "Midwinter Memento," granting ferrokinesis to manipulate metal. He used this power covertly to control weapons, mannequins, and metallic objects. Despite his abrasive front, he demonstrated loyalty to orders and comrades, attempting to dissuade Ichiyō Higuchi from defying Mori to save Ryūnosuke Akutagawa and later aiding her rescue. He also showed protectiveness, tending to his injured Hunting Dog colleague Teruko Okura.

His dual loyalties shattered during the Hunting Dogs Arc. After rigging a Port Mafia escape route with a fake explosive, he betrayed Black Lizard colleagues Gin and Ryūrō Hirotsu, gravely wounding them to preserve his cover. He then revealed himself to Yosano as the Hunting Dogs' spy and his brother's avenger, disclosing his mission to monitor Mori. This unmasked the calculated, rational strategist beneath his manufactured impulsiveness.

Tachihara's conflict reached its peak upon discovering Ōchi Fukuchi, the Hunting Dogs' commander, was the mastermind behind global terrorism as leader of the Decay of the Angel. Rejecting Fukuchi's manipulation, he denounced his actions despite years of unquestioning obedience. Blinded by Fukuchi in retaliation, he defiantly declared himself "the Mafia's Michizō Tachihara," symbolizing his repudiation of the Hunting Dogs' corrupted ideals.

As a Hunting Dog, Tachihara underwent light enhancement surgery, granting him durability against explosions and gunfire, though weaker than his unit's due to his spy role. His signature nose bandage, chosen by Saigiku Jōno as a "switch" between identities, represented his psychological compartmentalization. After his betrayal, he continued wearing his brother's dog tag beneath his Hunting Dogs uniform, a testament to unresolved grief.