A KEYSTONE Character is a character in a story who is essential to the plot and without whom the story would not be complete. The term 'KEYSTONE Character' is derived from the architectural term 'KEYSTONE, ' which is the central stone at the top of an arch that holds all the other stones in place. In literature, the KEYSTONE Character is the one who holds the entire plot together. The acronym KOP stands for 'KEYSTONE of Plot, ' which is another way of referring to the KEYSTONE Character.