Mitsuko Uzumaki is pein's/Nagato's little sister. Mitsuko is the reincarnation of a Phoenix demon that was killed on the day she was born, so in short she's sort of a tailed beast with a human form, that she prefers. Mitsuko and pein/Nagato where separated when the third shinobi war reached Uzushiogakure, mitsuko was three and pein/Nagato was 10, but they met again when Nagato met konan and yahiko cause they had found Mitsuko first. She was later taken to the mist where she became a ninja sword man at 7. And now where the story starts, she was sent to konoha to help establish a peace treaty between the mist and the leaf. I don't own naruto or the videos and pictures I may use.