Yuichiro had lost his arm because of the demon which attacked him and his twin brother Muichiro. He laid weakly on the futon and tried to stay awake, even when he lost a lot of blood already. As the sun began to rise Muichiro came back. He looked in shock and traumatized. He was covered in blood. Suddenly Yuichiro felt Muichiro's hand in his. Yuichiro spoke up and told his twin brother how much he liked him and he always only wanted to protect him. He said it because he was sure he will die today.

Days passes and Amane pays a visit again to the twin brothers with her daughters. As they saw the scene they immediatly helped them. Well...only Muichiro since they thought that Yuichiro was dead.

After they left. Yuichiro laid there alone and at the brink of death since had a little life in him. As the sun was gone and the moon was rosen, walked Muzan out of coincidence by the house and he sees the young boy. He feels that this boy was one of the heirs of the fog demon slayer and he felt that there was little life left in him. He picked Yuichiro up and ported with him to the infinity castle and turned Yuichiro into a demon. Muzan had of course a cruel plan since he knew Muichiro will become a demon slayer and Yuichiro were now a demon. He wanted to bring them together one day, but only to enjoy when they can't be with eachother anymore. Muzan gave Yuichiro to the uppermoons and gave them the task to make Yuichiro into an uppermoon demon.

3 years passes

You were with Muichiro, Tanjiro, Giyuu and Inosuke on a mission. You fought against some demons till you came to the uppermoon demon. You only saw how Muichiro freezes and his eyes widen in shock. Tanjiro and Giyu had the same expression and Inosuke shouted: ''Huh why does he look like you, Muichiro?!''. As you looked at the direction where the others were looking at, you saw Muichiro? But as you checked you saw that Muichiro was next to you. Who was this person? Had Muichiro a twin brother you didn't know about? Or was it just the demon his demon art?