a person whom one knows and with whom one has a bond of mutual affection, typically exclusive of sexual or family relations
a person's closest friend
A true friend is a friend when is convenient and when it is not. They standby you consistently both when you are present and when you are not. They're Authentic and Honest with You. True friends aren't phony with you. They show you who they really are.
A Friend for Life
A person that is willing to stay no matter how flawed you are, might get angry or emotional at times but is still willing to stay even if you block them out, even if you don't have any treasures in life. A friend for life are people that can make you happy and be happy with you. And would be there not just on your brightest days but also on your darkest.
What it takes to achieve one? Who knows? Maybe you already have one... a true friend...
... A friend for life.