A Friend.
A friend is someone who understands you, who believes in you,who you can trust, who would never turn their backs on you, who would always be there for you, who is reliable, who makes you feel better whenever you are down, and the list goes on.
Picking  a friend is easy but a close friend isn't easy. There are friends and there are friends. There are friends for the moment  and there are the real friends. The real friends are the one's who would support you, who would not lie to you in the face but tell you the truth.
If you think you've got the "bestest" friend in the world put them to test and see the results that's a  challenge to you.