Yuna was a Japanese thief who lived on the island of Tsushima during the 13th century. The sister of Taka, she was born in Yarikawa, and she and her brother ran away from an abusive household before being enslaved by the Black Wolf and the Mamushi brothers. They later escaped from slavery and lived a nomadic life on the island until the Mongols invaded in 1274, when Yuna rescued the gravely wounded samurai Jin Sakai after the Battle of Komoda Beach, aided him in recovering his katana (which she had previously traded for food and medicine to care for him), and taught him how to strike the Mongols from the shadows.
With Jin's help, she rescued her brother Taka from Azamo Bay, and she later crafted a tale to explain Jin's superhuman abilities, crafting the tale of the "Ghost of Tsushima" to rally the villagers against the invaders. Yuna repaid Jin for his rescue of her brother by assisting him with his capture of Castle Kaneda and his rescue of his uncle, Lord Shimura, from the Mongols. Yuna later asked Shimura for help with fleeing Tsushima with her brother, and Shimura agreed on the condition that she help him and Jin recapture Castle Shimura.
Shortly before the assault, however, the Mongol leader Khotun Khan captured and beheaded Taka, and a grief-stricken Yuna reluctantly fought alongside Jin and avenged her brother's death. With nothing left to live for, as she had failed in her lifelong quest of protecting her brother, Yuna decided to stay on Tsushima and help Jin fight off the Mongols to avenge her brother. Jin later refused to scapegoat Yuna for his own dishonorable tactics in his war with the Mongols, even though Shimura warned him that he would be executed in Kyoto for violating the samurai rules. After Jin escaped from Shogunal custody, he reunited with Yuna and assisted him during the Battle of Port Izumi, with the two of them admitting that they could not live without one another and vowing to do all they could to defeat the Mongols.
After the Mongols were defeated and Khotun Khan killed, Yuna lamented how Jin, as the "Ghost of Tsushima", belonged to everyone and not her, and she parted ways with him as he went into hiding to flee the Shogunal forces. She later reunited with him to counter the Eagle Tribe, Yuna stayed behind to help the victims of the Eagle's poison while Jin left for Iki to stop Ankhsar Khatun, or "The Eagle." He later hallucinated Yuna's rescue of him, who was instead Khatun herself.