Initially operating as a minion for a masked antagonist, Mitsurugi is assigned to eliminate protagonist Ken Tamaki, the sole survivor of a family massacre. This mission positions him as Ken's primary adversary in their initial clashes. Mitsurugi's path shifts dramatically from foe to reluctant ally through a series of brutal encounters. He repeatedly falls in battle, dying only to resurrect in later episodes—perishing during a meteorite interception attempt and again when a bomb strapped to his body detonates, yet returning each time.

Shady laboratory experiments unveil his origins: a human hybrid infused with kendama DNA, engineered as a weapon for global domination. Within the lab, he confronts Tsururugi, a sentient kendama bearing human DNA, in a lethal duel where the loser faces annihilation. Victorious, Mitsurugi demands her remains be preserved; they are refashioned into his signature kendama weapon. Lab doctors erase these memories before his release.

A frequent combatant in kendama duels, Mitsurugi utilizes diverse tactics and custom gear. He secures a diamond kendama via a massive 20-year loan, only to see it destroyed in a clash with Ken. Undeterred by repeated losses, he challenges Ken relentlessly, simultaneously acting as rival and reluctant informant on the shadowy organization targeting them.

Mitsurugi and Ken forge a multifaceted bond, oscillating between rivalry, uneasy teamwork, and sacrificial acts. Mitsurugi trades his own life to revive Ken in one skirmish and later risks death to alert Ken of looming threats. These choices mark his incremental acceptance within Ken's circle, despite ongoing betrayals and disputes.