What to wear: I hope you're not allergic to latex. Then again, if you don't know a makeup artist who can deck you out in head-to-toe blue, just go to a costume store, get a blue bodysuit, and draw on some weird scales. You'll also need a red wig and yellow contacts.
How to act: You're Magneto's right-hand woman, and you would do anything for him. You can also shapeshift into other people. Again, don't try this at a Halloween party. It would be weird.