To improve relationships. Talk to the fortuneteller. Ask what you need to do to progress your relationship with Nel. If she says "Share more of yourself with her" you need to let her possess you more.
If she says to witness more events or progress through the main story, you need to progress through the main story.
To do that, you need to talk to the barkeeper to tell you what you need to do next. He may give more than one hint on what you need to do.
Keep on improving your relationship with Nel, and you will eventually get to a point where you'll be able to possess Candy.