One of the more insidious and quietly menacing villains from X-Men history (read: she doesn't have a costume), Cassandra Nova is nonetheless one of comics' most deadly antagonists. She manipulated Bolivar Trask's son into releasing a plague of Sentinels upon the mutant nation of Genosha and committed genocide before setting her sights on the X-Men. Created by Grant Morrison and Frank Quitely, Nova was introduced as the dark shadow of Charles Xavier, who turned out to be her twin brother - and boy do they have some twisted family history. Although Cassandra Nova's severely reduced consciousness is currently housed in a harmless slug body in the comics, the producers of the X-Men movies are missing a trick by not utilising her potential to tell a story that rocks the foundations of Xavier's School For Gifted Youngsters. Not only would Charles' leadership be called into question, but the morally ambiguity and raw, ugly drama of Morrison's original story would be incredible to replicate onscreen.