Notes from author
Mikoto was born into a highly praised family of Astora. Her father, who's name was Oscar, was the highest rank of royalty, and her mother was largely praised individual due to her contributions to the kingdom. From an early age, her father trained her to be the best swordsman ever to walk the kingdom, and her mother taught her miracles from the age of four. She became one of the most honored individuals in Astora; however, at the age of seventeen, the curse of the Darksign began to spread. Oscar was branded with the Darksign and was exiled to the Northern Undead Asylum. Mikoto was outraged, and had a tantrum in front of nearly half of the kingdom. The king thought she had gone Hollow, and sent her to the Asylum with her father. However, her father had escaped his prison cell, and let her free; little did he know that this would cause him to be smashed through the roof by the demon that guarded the Asylum. Mikoto met her father in the room he had fallen into, and was gifted with an Estus Flask, and the key to escape the prison. Her father died soon after giving her these items, and Mikoto began to murder everything and everyone in Lordran to avenge Oscar; until she met Gwyndolin. Gwyndolin promised to reward her upon obtaining Souvenirs of Reprisals, and gave her a Darkmoon Blade. Now, she invades other undead like herself in Anor Londo and the Kiln of the First Flame; but she no longer kills to avenge her father, she kills for religion.
tl;dr: astora noble, her dad is Oscar, exiled even though she isn't undead, invade in Anor Londo and the Kiln.