The true friend
One of the main reason of calling someone a friend is because that person will stand behind you when no one will. The biblical tale of Jonathan and David is a candid example. Friendship requires loyalty from all the parties involved. Persons who are this type of friend, give their help in times of need.
Some people choose to become friends because of what they are to gain. So as long as they are gaining from the friendship, they stick with the friendship. When the goldmine which is the yardstick of the friendship is removed, they gradually exit the friendship to look for another source of goldmine. The goldmine which holds the friendship is usually material things.
Betrayal is the backbone of this friendship. This friendship has a lot of outward pretense which mimics a true friend. Following the outward pretense is backbiting, jealousy and envy. So there is always a chance to have those in the friendship to plot against each other at their back.