Why do some characters in Paulo Coelho's The Alchemist disappear and how does this reflect in the story?

As one
looks back over the story of Coelho's , it would seem that each character
in the story serves a purpose in Santiago's life journeyas he seeks the truth of the worldhis
Personal Legend.

King Melchizedek (the King of Salem)
teaches Santiago to look for what lies hidden from the eye of others that do not have the heart
to see what is there. He tells Santiago about persevering: that have coming so far, he must not
become discouraged, or he will never realize his Personal Legend. The King of Salem teaches
Santiago the need to be wise:

Everything in life has its
price.

Santiago loses everything to a thief and cannot
continue on his journey. He agrees to work for the crystal merchant, even if only to earn enough
money to return home with some sheep.

As Santiago works for the merchant, he
learns that the older man is devout in his faith, which demands five obligations of each
man. 

The fifth obligation of every Muslim is a
pilgrimage. We are obliged, at least once in...