Who is the main antagonist of Book 1: Air?

The main antagonist of Book 1: Air is Amon. He is the masked leader of the Equalists, an anti-bender revolutionary movement that challenges the government of Republic City and argues that bending has created an unfair social hierarchy.

Amon claims to be a non-bender whose face was scarred by a firebender and says that the spirits granted him the ability to remove bending permanently. His campaign quickly grows from public speeches and attacks on bender-controlled gangs into a coordinated assault on Republic City. This makes him the central threat faced by Korra during the first season.

A major twist reveals that Amon is actually Noatak, the son of the criminal waterbender Yakone and the older brother of Tarrlok. He is secretly an exceptionally powerful waterbender and bloodbender, using bloodbending to block a person's chi pathways and imitate energybending. The other options are important antagonists from later books: Unalaq appears in Book 2, Zaheer in Book 3, and Kuvira in Book 4.