I'd like to propose a character as well.

A water bender, and army brat child of a Major from the Northern Water tribe, he originally entered the military like his father. However, when Iluq took power, he found himself disagreeing with her goal of creating a world ice age, especially when he saw other peoples, especially benders, suffering for it. Climbing the ranks, he eventually became a special operations agent, part of an elite team of assassins and recon / spies working under Iluq and her generals. With this position, he began to funnel vital information to the Fire Nation military, to try and subvert the Water Tribe's forces.

Unfortunately, a spy within the Fire Nations army, working for Iluq directly, found out about the deception, and attempted to assassinate him, which resulted in the assisin believing he had died in a burning building. However, the corpse found wasn't his; having managed to escape, he went into hiding, awaiting his chance to strike back at the rapidly growing Water Empire.