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.
When Yuna and the party were declared traitors to Yevon after killing Maester Seymour Guado, squads of warrior monks pursued her across Spira after the party escaped Bevelle. The subsequent commotion caused by Kinoc's death, Kelk Ronso renouncing his maesterhood and Seymour disappearing after being exposed as a patricide and an unsent, there were riots in Bevelle which forced Mika to call back most of the warrior monks to quell the unrest, abandoning the other temples throughout Spira.
By the time the party returned to Bevelle, Mika defused the situation with Yuna by proclaiming that her being declared a traitor was a malicious Al Bhed rumor. Once Mika sent himself, the monks remained in Bevelle to defend the city from Sin.
After Yevon's corruption was exposed a number of Warrior Monks considered quitting the organization and some even joined the Crusaders.
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.