Based on the erotic game by Elf.

After Kisaku died, he had a strong desire that his spirit was still wandering around. Yuri noticed a guy who resembled Kisaku, but he was actully a hard worker named Kusaki, who had a good nature and was far different from Kisaku. When they first met, Kisaku's spirit appeared and started to haunt Kusaki. Eventually Kisaku took over Kusaki.

(Source: AnimeNfo)