Joe Otterson TV ReporterThe “Avatar: The Last Airbender” live-action series at Netflix has cast three key roles, with production on the series now underway.Paul Sun-Hyung Lee (“Kim’s Convenience,” “The Mandalorian”) will play Uncle Iroh, a retired Fire Nation general and the wise and nurturing mentor to his hot-headed nephew, Prince Zuko. Lim Kay Siu (“Anna and The King,” “Nightwatch”) will play Gyatso, a kind and caring Air Nomad monk who is the guardian, father figure and best friend to Aang.