A best friend is someone you can trust with anything who cares deeply for you and wouldn't let anyone hurt you or be by your side when someone does they will not leave you when you are going through a tough time and they know when something is wrong with you even when you are trying your best to hide it. They are often the voice of reason when you are being a dick and they are very honest with you unless they know not to be and hold back there opinion for your sake. They are often crazy and the only person you can be a total and utter freak with and be comfortable about it. They are your substitute soul mate.
Best friend: hey, how are you
Me: I'm great I couldn't be better
Best friend: no, there's something wrong with you don't try and hide it.
Me: OMG how can you tell