I am trying to write up a character (Belladonna) who appear human but her actions give her away her true nature. She is the incarnation of a entity from the void who only view humans as toys and fuel for her magic (she feeds on their souls.) Also biologically she isn't alive and can only however she can fake bodily processes. The plot requires her to establish some long lasting contact with a human in order to drain them. I've thought making her emotionless but that has been done all over the place. Any suggestion?

There's been a lot of threads on this in the past. I'd recommend that you check these ones out first and see if they help you out at all. If you have any other questions relating to this, go ahead and post them here. If you have other, unrelated questions, try either of the search features located above the banner ads on forum pages. Note that the one on the left, labeled 'search forum titles' looks for the exact string that you put in, and only looks at thread titles.

Doesn't use the bathroom but eats

Take a look at this.

Insects and other big creepy crawlies are scary because they move in ways we as two legged, two armed creatures are uncomfortable with. Your character could have a gait or micro-expressions that are just so off. It could move under clothes in a way that suggests that it has more joints that normal or that the clothes are jut a costume. Better yet, her skin could appear to be a costume, getting baggy/saggy at times, other times taunt and appearing to hide something shaped like a person.

Summarizing most previous thought on the topic: focus on the otherness.