Jinzaburō Kuchii, once a samurai retainer of the Kamakura Shogunate, faced disgrace leading to imprisonment and exile on Tsushima Island. This punishment persisted despite his master's posthumous pardon and followed profound personal loss: a plague claimed his family, including a child, deepening his disillusionment. Arriving on Tsushima, Kuchii and fellow exiles were unexpectedly recruited by Princess Teruhi of the ruling Sō clan to defend against an imminent Mongol invasion. Initially resistant, Kuchii intervened to stop an assassination attempt on Teruhi, marking his shift towards commitment to Tsushima's survival, an island with which he had no prior ties.

Kuchii wielded exceptional martial skill, mastering the Gekei sword style associated with Minamoto no Yoshitsune. This style emphasized battlefield strategy and acute situational awareness, enabling him to assess chaotic situations and command effectively. His tactical insights proved vital for the defense, often clashing with clan leader Sukekuni, whose complacent reliance on outdated tactics endangered Tsushima. Kuchii openly advocated for pragmatic, adaptable strategies against the Mongol threat.

After Sukekuni fell in battle, leadership of the Sō clan fractured between Teruhi and her adopted brother Abiru Yajirō. Recognizing the critical need for unity, Kuchii mediated their conflict. He further forged the remaining exiles—including former adversaries like the pirate Onitakemaru—into a cohesive fighting force, earning their trust through shared hardship.

As the invasion intensified, Kuchii led civilians and fighters to Kanatanoki Castle, uniting with the Toi Barai unit under Nagamine Hangan. His leadership faced internal threats: the exile Obusuma Saburō conspired to betray Tsushima to the Mongols. Though Saburō was killed by fellow exile Shiraishi Kazuhisa, Shiraishi himself later defected, revealing castle weaknesses to the enemy. These betrayals highlighted the defenders' desperation, yet Kuchii persisted in rallying them against overwhelming odds.

His motivations evolved from seeking personal redemption to dedicatedly protecting Tsushima's inhabitants. He coordinated guerrilla tactics, leveraged comrades' diverse skills like Amushi's scouting and Hitari's archery, and sustained morale despite fading hopes for reinforcements. His resolve earned him a formidable reputation; some perceived him as a figure of mythic resilience, the "Sun God of Tsushima" (Tendou Houshi). While his identity remained rooted in his past as an outsider and prisoner, his actions defined him as a pivotal force in Tsushima's struggle, embodying the transformation from displaced warrior to unwavering guardian.