The (ironically) fiesty waterbender who is determined to improve her bending evolves into a young woman who is stronger than almost all waterbenders due to learning bloodbending and is able to use her compassion in order to genuinely move and unite different types of people.
The worried teen in charge of his entire village in the absence of his father's presence evolves into someone who has great confidence in his abilities and is able to work as a compelling leaders, creating plans that help his bending friends
A boy running away from power that people would die for evolves into a human being who faces his challenges head on and never backs down when it comes to his morals.
The angsty, banished teen who lives for his father's approval evolves into a young man willing to forsake all he's known and loved in order to do what is right.
The wise old man never really evolves within the series, but the audience's perception of him changes drastically throughout the plot as his relationship with Zuko, his nephew, changes.
Zuko's sister puts on a persona of being a strong woman, ready to take down any who get in her path, reveals herself to be a lonely girl who just wants her father to love her and will do anything to gain his love.
Ozai serves as the leader of the fire nation and works as a villain to nearly every character in the form as himself as a father and brother or in his more well-known form as the leader of the Fire Nation.