Respect. If they respect you and you respect them, that is a beautiful friendship. However if you both like to hang out, but they do something to disrespect you or something you believe in, thats when it becomes unhealthy. You will be lying to yourself or "sitting down" and just taking beatings in your head whenever they talk bad about something that is dear to you. Let your concerns be known and if they still disrespect you, that isn't a friend.
Honesty, always being able to rely on her, never judgemental, being able to trust her completely, being a great listener, giving good advice, telling me I'm wrong if I'm wrong, but always having my back. Being a great shoulder and able to have fun