The Warrior Monks are an army of Yevon and often serve as bodyguards for the Maesters of Yevon in Final Fantasy X. Their leader is Maester Wen Kinoc. According to the Final Fantasy X Scenario Ultimania, the monks' principal duties are the defense of the temple of St Bevelle and the protection of high ranking priests. They don't play a fundamental role in security around the world, although they are deployed throughout Spira in times of emergency. Officially, the Warrior Monks are considered to be a moderate organization, but in secret they crack down on those deemed heretics.
The warrior monks' appearance has not changed much since the Machina War. There appears to be two classes of monk; one class wears a green uniform with armor and another class wears a red, orange, and yellow uniform. The Warrior Monks' weaponry—a rifle and a flamethrower—also has not changed since the Machina War, which is notable considering that machina weapons are forbidden by Yevon.
The history of the Warrior Monks date back at least 1,000 years prior to the events of Final Fantasy X. In the Machina War, the Warrior Monks were under the control of Bevelle and were used to fight Zanarkand. Many monks died during Sin's destruction of Zanarkand, and many Fallen Monks still dwell there as unsent.
Both Auron and Wen Kinoc were once Warrior Monks in their youth. While Kinoc steadily rose through the ranks to eventually become Maester, Auron was thrown out after refusing to marry the priest's daughter. He then went on to become one of Braska's guardians and defeated Sin.
Two types of Warrior Monks are fought in Bevelle; one wielding a rifle, and the other a flamethrower. Warrior Monks can be fought later in the game with rifles and flamethrowers again, however the Fallen Monks in Zanarkand Ruins are undead.
The Warrior Monks during battle only fire with single shot rifles, but in the cutscenes, they are shown with the same rifles shooting automatic rounds. One explanation is that the weapons have selective firing modes, as they do in real life.