Mio first appears halfway into the anime series. She is a character exclusive to the anime version of D.N. Angel, a student who suddenly transferred to Daisuke's school (as shown in episode 14). All that is known at that time is that she left for America and stayed there for 2 years before coming back to Japan. First impressions of Mio is that she is a rather simple minded "valley girl" (or an American gaijin in the Japanese version, even going as far as to say "Sorry! I can't speak Japanese!" when confronted with certain demands or, in the English dub, telling Riku in Spanish that she can't speak English.
It is later revealed that she is actually a voodoo doll living in a temporary body and promised a real life by Satoshi Hiwatari's adoptive father, Kei. However, in order for her to attain a real life, someone must be sacrificed... in this case Daisuke. The initial reason why she occupied herself with Daisuke was to get him to wear a powerful magical charm (actually a pair of charms [one in the shape of a heart, the other in the form of a heart shaped key] and the pair must be interlocked when worn for the magic to activate) that would then seal his soul within it. However, over time she eventually fell in love with him. She wanted a real life, but not at the cost of Daisuke's. Mio then decides that she refuses to give up Daisuke's life and ventured to retrieve the magical charms from Riku (whom she had given to in hopes that she would get Daisuke to wear it). Despite her actions, she arrived too late at the Harada house seeing Daisuke and Riku out in the backyard, both with the magical charms around their necks. Unable to stop them, she used her powers to negate the charm's effect and sealed herself within Riku's charm before Hiwatari's father was able to negate her temporary life. Before the series ended, Mio's spirit (sealed within Riku's charm) pleaded with the Harada twins to lend her their power so she could save Daisuke and Dark as well as Hiwatari's father. She then gains her own body back, using the Harada twins as a medium, and goes out to prevent Kei Hiwatari from activating the Black Wings in hopes of saving his life as well. Retaining some of her powers and using that of the Harada twins, she was able to stall Hiwatari from activating the Black Wings she was, however, unable to prevent his death. As the last of her power was used up, she left, telling the twins that she was glad to be able to do something to save Daisuke and Dark and requesting that they thank Daisuke for her, since it was through him that she was able to truly be alive. Her last words before disappearing in a shimmering light were: "Riku....you and Dai.... are an uber cute couple!!" (or in the Japanese version: "Riku...be happy with Daisuke, damn it!")