Mewt is a main supporting character in Final Fantasy Tactics Advance. In the real world, he is a shy, unassuming boy (assumedly teenaged) who lost his mother at an early age. He always carries around a teddy bear.
Mewt is unhappy with his life in the real world-he is always the butt of jokes and has nearly no friends, save for Marche and Ritz. His father is almost completely broke, and was just recently near-fired at his most recent job-since he was laid off by his previous employer and his wife's death he steadily lost interest in most everything.
When Mewt finds an unique book in an antique bookshop,he takes it to Marche's house, where Marche, Ritz, and Mewt try to read it-but they can't. One of them suggests the book "looks like Latin"... and then the topic turns to video games and what it would be like if the world was one. Final Fantasy is mentioned, and later that night the town of St. Ivalice is transformed into Ivalice, all under Mewt's whim.
Within Ivalice, everything is perfect for Mewt: his mother, Remedi, is back as Queen, his father is Judgemaster of all of Ivalice, and he is the crown prince, with his own personal servant. Even after Marche destroys all the crystals, Mewt's will alone keeps Ivalice from imploding. Mewt finally understands that his escapism of staying in Ivalice will prevent him from accepting the reality of his own life. Upon the defeat of Queen Remedi he voluntarily leaves Ivalice.
Source: the game