Evil is as evil does. Cassandra Nova is Professor X's lost twin who died in the womb and is responsible for the 16 million mutants that were slaughtered in Genosha. When Emma trapped Nova in Stuff, she was able to leech onto Emma's subconscious and guilt her into believing she was always evil and even created an imaginary Hellfire Club in the process. Through Emma's created reality, Nova planned on finding a new mutant host to probably screw over the world some more!
After 18 or so issues, Emma defeated Nova, though her whereabouts are unknown... Emma did tell her to go to hell.