The Geomancer is required in order to win The Thief of Fate, but only because of the WAWA spell. You should generally not try to use the Geomancer for heavy-duty spellcasting, but instead, keep him as a front character. The most common candidates for conversion into a Geomancer are the Paladin and the Warrior. All classes can become Geomancers, but they lose their former special abilities.

Only Umbrech in Kinestria can teach a character to become a Geomancer.