The Playback Miko's real name was Riria. Like the other Mikos, the name was taken from her and she became a tool for the strong. Her power was the ability to to rewind time to a maximum of 24 hours.
The Playback Miko was born with a blank expression, similar to the dead.
Due to her fate of being forced to repeat the same life continuously, the Playback Miko realized her fate in every timeline would only be slightly different, and would always end in her horrible and early death. Each time she died, a new nightmare. Eventually, her mind breaks and she becomes a vacant and obedient machine with one continuous reel of her death playing.
She was born in the Armored Dragon Calendar Year 500. Regardless of the timeline, the Playback Miko is born in a remote countryside and is shunned by adults and children alike. At the age of 5, soldiers would take her away. With some variation, she is usually captured, imprisoned, and passed between ruling powers until she dies at the age of 10, or earlier.
Volume 24 - Final Volume - Conclusion Chapter
A girl is born with the power to rewind time of an object to a maximum of 24 hours. She is able to divert death, and disease, as such, she is coveted by nations. Regardless of her abilities, no matter what she did, she could not change her faith. Over the course of centuries, she attempted to change her destiny, but could never end the hellish loop of her birth and death. Eventually, she would reach a horrible death, watching herself be beaten to death, eaten, massacred, and dying shackled as a puppet to the strong.
In one playback, she is used in a magic formation and a boy, Shinohara Akito is summoned. The boy was different, and her world changed. She becomes his interpreter and they fall in love. During the battle with an enemy Kingdom, he was beheaded and by the time the Riria found him, 24 hours have already passed and it was too late for her to revive him.
Due to the emotional impact of losing Akito, Riria opened a rift in time and space in the past so that she and Akito could live together. She failed, but an unrelated soul was able to pass through the rift. This was Rudeus's soul which happened to be near Akito in the other world.