A human who serves as a bully and possibly romantic rival for Riley.

I was thinking about an evil woman rejecting her emotions.

Maybe an emotion called Anhedonia, trying to suck the emotion of other emotions so she won't be the only one who feels nothing.

Maybe Riley is going through a lot (like depression), causing Anhedonia to appear. I know this won't make a good "kid's movie" but it would make a good book ;)