Quick summary:
Sayaka is bed-ridden and dying in hospital, but she possesses popular girl Sakuragi Erika and uses her body to be with her beloved boyfriend Naoto. But she has yandere tendencies. When Naoto jokes that maybe she's not really Sayaka, she decides to prove it by stealing his body and putting Naoto in Erika's body. Her power allows her to overlay a person over the original owner, inheriting their memories and such. Furthermore, if a person orgasms in that new body, they forget their original self. She then forces Naoto into becoming Erika, and later many other girls, including his own little sister and mother.

Sayaka reveals that she both loves and hates Naoto, jealous that he can run around healthily while she's dying in hospital. Her original body later passes away, while she continues to possess Naoto and other girls. Naoto later uses willpower to remember being himself, and eventually realizes he enjoys being a girl. He asks Sayaka to conspire with him and take over various girls' lives, to which she obliges.

Last I checked, it's currently 5 chapters (chapter 5 description says Sayaka has something nasty planned for Naoto), and IIRC Marialite said there's a finale in the works.

Base +6, ensane +9