Abiru Yajirō, adopted son of Tsushima Island's Lord Sō Sukekuni, occupies a key position within the So Clan's leadership hierarchy. He shares strategic responsibilities with his adoptive sister, Teruhi. Initially skeptical about their defense strategy, he opposes integrating exiled outsiders into their forces to face the Mongol invaders, distrusting reliance on criminals.

The death of Sō Sukekuni during the Mongol assault shatters Tsushima's forces, plunging Abiru Yajirō into profound demoralization. Witnessing his father's loss and the army's complete defeat, he questions the purpose of continued resistance. Constant retreat with survivors across the island, finding no safe haven, deepens his despair. He grows weary of relentless fighting against overwhelming enemy numbers, contemplating endless suffering and an uncertain future.

This despair culminates in a pivotal shift. Abiru Yajirō confronts their dire circumstances, resolving to fight with every ounce of strength until the end. This decisive acceptance rallies the remaining survivors' morale, providing a focal point for continued resistance against impossible odds. His transformation from doubt and weariness to resolute defiance marks a key development in his narrative role.