Reishō Miketsu/Yoshie Shiniyukumono

Yoshie Shiniyukumono (死にゆく者 淑恵 Shiniyukumono Yoshie) is the Plus of a deceased Human girl who resided in Yasuraka Town, attending Yasuraka High School. Being possessed of great spiritual power for a Human, Yoshie was capable of shocking supernatural feats in life. Due to a set of unfortunate circumstances, she was killed, and came to possess the body of Reishō Miketsu, merging her soul to his. As such, she is capable of wrenching control of his body from him to use it as she sees fit, and can channel her powers through his body. To make it more comfortable for herself, Yoshie alters Reishō's gender to a female when she is in control, as a female body is the kind she is used to.

Because of the nature of her existence, Yoshie is classified as a Tsukirei  ( 憑き霊, Possessive Spirit ) by Soul Society.

Trivia

 * Yoshie's surname, Shiniyukumono, literally means "dying person".
 * While Darkrai has no proof of this, Yoshie is most likely the first Tsukirei in any BLEACH media, fanfiction, canon, or otherwise.