Who is Juan Alvarez from The Moon Is a Harsh Mistress and what is their importance?

The Security Chief who commands the disgruntled Luna Authority Militia, Alvarez is an ex-convict and the loneliest and most frightened man in Luna. When his spies penetrate the revolutionary organization and are killed and all trace of their existence disappear, Alvarez over-reacts and the populace grows restive. It becomes unsafe for Yellow Jackets (militiamen) to patrol singly. Mishandling of a rape/murder in Luna City precipitates a coup in which Alvarez and all of his guards perish.