Raiko is the physical incarnation of Daichi Thorne Takeya’s zanpakuto, Aoi Raiko. During Fifth Division in the Red, she incarnates as Raiko, a blue haired girl and calls herself Daichi’s younger twin sister. She calls Shirahime “mama” and is considered her youngest child. She remains in physical form most of the time.
Shirahime’s family welcomes her but sometimes hide her from others and many shinigami wish to destroy her.
She is closest to Shirahime, Daichi, and Shinjiro Kita.
SPOILERS FOR FIFTH DIVISION IN THE RED BELOW:
Raiko is actually Daichi’s younger twin sister that Shirahime hid inside of him so that their biological father (simply known as “the bad man”) did not find out about her. Later on, the family lets others believe Raiko is just a zanpakuto spirit to keep her safe. She considers Shinjiro Kita to be her father though she also calls Crown Prince Shiroyuki her father. She is well-versed in the customs of the royal realm.