@emilyandersons no, the deal with the devil is that she gets her old life, in the real world, back, if she can fulfill the countess' wish of happily marrying the duke. If she fails, she just dies as she was supposed to when she got stabbed. It does leave some questions up in the air (what's happening in the real world for those six months? What happens to her real body? etc), but basically it seems the devil is using her to fulfill the contract he had with the countess, who died for some reason just after signing the contract.
The sunfish game... is talking about that one game, isn't it? I know it was pretty popular for awhile in Korea. I wonder what kind of ending this would have if the body does die after half a year+.