Not much about Mio's past was revealed. It was shown that she met the Fudō twins, Sigma and Leo, during their training days as potential heirs to the Makai Armor Lord. Apparently, Mio developed feelings for Sigma and went searching for him after he ran off due to losing the title of Flash Knight Lord to Leo.
After successfully finding Sigma, she surprised Leo by returning with his brother and the trio was back together again. After reuniting, Mio left with Sigma to help him realize his ambition, creating Idea. She agreed to help him research and develop such a machine to help rid the world of darkness and was instrumental in discovering a mystical process to generate enough power to realize Idea.
Through a passage in a book she found, it mentions a means of merging a Makai rock with a soul, to generate infinite power. With that knowledge, both Mio and Sigma went to search a great power source for Idea; the two found the corpse of Gyanon, the Fang of Messiah. Mio had her doubts about using a Horror's corpse as Idea's power core and went to see Leo about the matter. Leo showed his inventions to help take things off her mind and reassured her that Sigma isn't doing anything; he offered her sanctuary at his home if she feels differently.
Mio decided to settle things with Sigma, but when she found him, she was shocked to find Sigma had killed a woman (suggestively a prostitute) to experiment on. Sigma tried to calm her down, explaining that the woman is a sinful person and wasn't worthy of life. Angered and disappointed, Mio ran off to find Gyanon's body to sabotage Sigma's project. However, Sigma wouldn't allow it and got in her way. Mio tried to destroy Gyanon's corpse, but Sigma wouldn't be denied and killed her. In the wake of her death, Leo found her body and mourned her death. He took her brush and suggestively buried her. Mio's death would be the turning point for Leo to take arms against Sigma.
During the great battle Sigma, Idea was realized, but Gyanon had reawakened and assimilated Idea as its new body. Utilizing the awesome power of Idea's abilities, Gyanon intends to infest the mortal realm with Horrors to feast upon. However, Leo had an idea: using the magical powers of the brush, he can pass it to a Makai Knight to turn it into a powerful magical arrow, The Shooting Star of Light Arrow. Using Mio's brush, he asked for her power and used it to stop Gyanon. Afterward, Mio's spirit reveals herself, showing she was happy for Leo's victory and bid him farewell.
- Makai Priestess Training: ?.
- Madō Brush: ?.
Notes & Trivia
To Be Added