A true friend is someone who would always be there when you needed them the most. Someone who would lend a shoulder when you feel tired, never lie to you, and help loosen your burden. I think that is the most important quality that distinguish them from others.
When a problem arises or at a time we feel helpless and depress, talking about our problems help lessen our troublesome emotions. But in times like this, we just wanted someone who listen without interrupting or lecturing you about how you should cope with it. A person who would just listen and try to understand because they genuinely care.
However, if you are in denial about your problems, a friend definitely will not stand aside and let you continue your destructive behavior. She would tell you the truth even when it is gruesome. Doing this may hurt you or bring unhappy thoughts, but she would say it, regardless, to help you see through your repudiations.
Most of all, she would always assist you every steps of the way. Like when you want to give up, she would encourage you to continue, and help you get back on track. You can always depend on her, because she will never leave you at the bottom of the pit and walk away.
Therefore, it is rare to have someone being there for you when you require their support the most, because not everyone is willing to waste their time and energy. There was a quote that I read that perfectly describe that type of person: A true friend is one who walks in when the rest of the worlds walks out.