Is someone who

Would step into your life at the right time, back off when it isn't right.
Listen to you, instead of deciding for you.
Give you the frank statement instead of telling what you want to hear.
Try to ease your pain instead of creating hurt.
Support you to do the right thing, and would give the reminder when you are not.

These are the friends we should not disappoint.
The rest, we can choose to put them out of the picture of our life.