fifth pillar
Someone who, having intimate knowledge of a company, takes action to destroy that company from the inside.
A certain corporate bookstore treats their store-level employees like they are worth less than dirt. JoAnna, a long-time employee of said bookstore, is sick of corporate abuse and no longer cares whether or not she receives minimum wage to sling coffee. She's the fifth pillar - she will do everything in her power to destroy this bookstore with the knowledge she has as an employee.