Niiniu is the last of a people who heal other hearts with their songs. She was able to sense Ruruta's pain and insecurities as he was unsure whether he was truly the world's savior, and promised to forgive him should he fail in his duty, even when the world wouldn't.
They became lovers, and this new bond considerably raised Ruruta's strength, as he vowed to protect the world she loved.
However, Ruruta's followers objected to their affair and lured him into drinking water from one of the seven war machines, Argax, to erase his memories of Niiniu. They proceeded to severely beat Niiniu, crushed her voice and buried her half-dead. As Niiniu was about to die, the God of the Future, Orntorra, appeared to her and convinced her that the human world was beyond saving, and that destruction was the only means to prevent humans from creating such world ever again. He made her soul the core of the Beasts of the Final Chapter, which Ruruta was to face in his final fight.
When Ruruta finally reached her and killed the Beasts, he was given her book by Lascall Othello and so remembered Niiniu. Desperate to give her happiness, now that she had shed her human heart and desired for destruction only, Ruruta ate her book, destroyed the world and built Bantorra Library. He himself became Heaven and fed on countless books in the hope that through these books, Niiniu could find happiness. However, in the final battle, when Niiniu awakened inside Ruruta's stomach after he gave her his power, she blamed Ruruta for making her kill everyone whose souls were still alive inside him, and vowed to never forgive him.
