Someone who can be cool, but is often
calculating your every move
on the inside. They can be deceptive and decietful, and appear to be all knowning at times because of people's predictability.
They often prey on your weaknesses, and notice all of your flaws just by looking at you.
However because of their ability to see
indefinite flaws, they wind up hating themselves, because they see all of their own flaws in extreme detail.
In general, "Viper" would be the name for your sneaky, slightly badass, tough friend who's skilled at many things the average biddy would never dare to try.