Yandere Kanojo ni Nemurasete Moraenai Hanashi

A high school girl lost her beloved boyfriend, Rei, in a car accident. In the midst of her despair, she manages to successfully transfer Rei's soul to a teddy bear. They had dates together, gave gifts to each other, and although his appearance was different, they were happily living together. However, something was gradually starting to change within Rei...

(Source: Kadokawa, translated)