A good friend is honest. A good friend may not share every detail of every second of their life, but they do try to be clear about their intentions. This means that they try to present an accurate picture of who they are and of different situations. When something doesn't seem right, they let you know.
A good friend is fun, unique and interesting.
this is a given, and probably the reason you became friends in the first place. But there's a lot to be said for chemistry and shared interests. As for fun, it depends how you define it: Some friends are fun because they're the life of the party, others are fun because they notice every strange little detail about a situation. Some people are fun simply because they see life like no one else does.
A good friend is a friend you can trust.
A true friend won't try to steal your girlfriend or boyfriend, your job or your personality. They won't gossip about you constantly or try to damage your reputation. They will let you know when they're concerned and do their best to stick up for you when you're in trouble.
A good friend sticks with you in good times and bad.
Loyalty is a quality almost everyone lists when asked what they look for in a friend. A loyal friend will stick with you when your new play is a flop, when you bomb the SATs or when your parents get divorced. If you move or switch schools, they'll do their best to stay in touch with you.