Miu is Sora's only family who had frail and sickly body, causing her to be hospitalized with Sora often went to visit her. Despite Sora's encouragement to be optimistic that she will be cured of her illness, Miu knew that she won't live long and told him to see the world in her place. She eventually died due to her illness, leaving Sora distraught. G.O.D responded to Sora's desire to be with his sister, allowing him to repeat his life in a loop. However, even with the knowledge of her death, Sora couldn't find a way to cure Miu's illness even with future medication, resulting Sora to repeatedly witness his sister's death in each loop. Once the contract with G.O.D was over, Sora made it his goal to restore G.O.D's true power so he can have G.O.D create a parallel world where he can be with Miu forever.
In the middle of the Action Duel between Sora and Yuya Sakaki, a photo of Sora and his sister flew from Sora and ended up on Yuya's hands. Yuya looked at the picture and asked Sora if the girl was his little sister. Upon hearing the question, Sora started shaking and shedding tears, saying he couldn't lose the Duel no matter what. He then explained that he had a sick little sister, but Reiji Akaba held her hostage, with the intent on killing her if Sora did not win. Sora begged "Phantom" to let him win the Duel, with tears flowing from his eyes, making Yuya choose not to activate his "Enter-Mate Show Up" Trap Card upon Sora's attack. However, Yuri switched places with Yuya and called him soft, declaring he wouldn't go easy on Sora. Sora tried to remind Yuri of his sick younger sister, but Yuri yelled that he didn't care, and that they could both die for all he cared.
As the Duel progressed, Yuri declared that he was lying. Noticing Yuri had saw through him, Sora revealed his true colors and admitted that the story was all a lie. Yuri then ripped the photo of Sora and his sister in several pieces. Soon, a candy Yuri was stepping on exploded, with Sora explaining that the photo was a trigger that would cause the candy to explode if it was destroyed. Yuri then noticed that was a countermeasure Sora came up with in case his opponent saw through his lies.
After Sora's defeat, his sister appeared in his dream, telling him to wake up and asking him if he had forgotten about her. Sora called her "Miu", and replied that he would never forget about her and that he swore that he would create a world where he can be with her, before waking up in a hospital bed. Isaac explained to Sora that he was there because of the injuries he sustained in his Duel against Yuya, and Sora noticed that he had manipulated his memories to infiltrate the Leo Corporation.