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.
I didn't notice how bad Jimmy's acne was until that Viper pointed it out.
She's such a viper, she looks all sweet but she's just using him for gossip.