Avatar ; the last airbender is ultimately about bringing piece, and balance to a war torn world.
Both zuko and katara embody the very characteristics of their respective elements. Zuko being seen as a complete, angry driven brute. And katara as a compassionate healer.
“Fire is the element of power. The people of the Fire Nation have desire and will, and the energy and drive to achieve what they want.”
“Water is the element of change. The people of the Water Tribes are capable of adapting to many things. They have a sense of community and love that holds them together through anything.”
The avatar in some way is like a “catalyst” for bringing these two opposing forces together; and in this act, sending the avatar universe into a course of events that will ultimately send it back into a state of balance.