Who is the Grand Inquisitor? How is he described and whose side is he on? Who is the "us" of whom the Grand Inquisitor speaks?

Who is the Grand Inquisitor? How is he described and whose side is he on?

Who is the "us" of whom the Grand Inquisitor speaks? And, how did they (the "us") elevate themselves and become different than the masses of humankind? 

What do you think Dostoevsky is trying to communicate about the nihilist perception of the divine through the parable of "The Grand Inquisitor"?