Bontenmaru, later known as Date Masamune, emerges as a young Christian samurai marked by distinctive mixed heritage within feudal Japan. Her heterochromatic eyes—one chestnut brown, the other blood-red—fueled childhood ostracization. Superstitious peers branded her a "demon child," while her mother Yoshihime declared the eyes "ugly," refusing to meet her gaze. Rumors suggested her birth resulted from Yoshihime's affair with a foreigner, escalating clan tensions and triggering Bontenmaru's half-banishment to Sakai. This profound rejection manifested in delusional convictions, including a fervent belief she was the beast of Revelation destined to destroy the world—a mindset termed "eighth-grader syndrome."

Her appearance features blonde hair, frequently concealed beneath an eyepatch masking her heterochromia, and attire blending black clothing with Christian adornments reflecting her faith. Katakura Kojūrō, her steadfast retainer, remains a constant presence since her youth.

A pivotal shift occurred when Bontenmaru encountered Sagara Yoshiharu in a Kyoto church during the Kyoto Arc. Attempting to repel him with imagined "seals," she was instead disarmed by his unexpected kindness. Yoshiharu inadvertently catalyzed her transformation by reframing her heterochromia as a unique asset ("Jakigan"), popular in the future. Inspired, she adopted the title "Jakigan Masamune," vowing to conquer Ōshu, overthrow her father, and challenge Oda Nobuna—whom she perceived as a rival "Demon King"—to determine Japan's true ruler.

Her Ōshu campaign involved constructing a non-Christian church, mobilizing forces under Kojūrō, and leveraging fears of her "supernatural" eyes for psychological advantage. Despite ruthless ambitions, vulnerabilities lingered, particularly concerning her mother. Yoshiharu intervened during her conflict with the Takeda clan, urging aid for Yoshihime against the Mogami. This forced Bontenmaru to confront deep-seated insecurities about her heritage and maternal rejection, leading to tentative reconciliation and a new focus on protecting her family.

Key relationships defined her journey: Yoshiharu served as an older brother figure, offering guidance while unintentionally stoking her ambitions; Nobuna acted as a foil, sharing parallels as fellow outsiders with strained maternal ties and European connections; Kojūrō provided unwavering loyalty, often tempering her impulsivity. Bontenmaru's arc culminated in consolidating Ōshu's power, balancing her apocalyptic self-image with pragmatic alliances and familial duties.