In the world of Final Fantasy X, the Maesters (老師, Rōshi?) are the leaders of Yevon, the dominant religion of Spira. There are three positions with the title of Maester; one is held by a Ronso, one by a Guado, and one by a human. At the time of the storyline of Final Fantasy X, the three Maesters are Kelk Ronso, Seymour Guado (who had just succeeded Jyscal Guado), and Wen Kinoc.
Each Maester is the leader of their people and their representative in Bevelle. They perform various duties to do with the governance and protection of the people of Spira, such as commanding the Crusaders and overseeing the Temples of Yevon.
The Maesters are not above keeping secrets from the people of Spira. For 1,000 years, the truth regarding the Final Summoning was kept quiet to preserve the spiral of life, death, and hope, which surrounds Sin. Although allowing citizens to know the truth may have served Spira better, they believed Sin could not truly be defeated and fostered the false hope in order to keep Spira going.