Azula

Azula  was the  princess  of the  Fire Nation, daughter of  Fire Lord   Ozai  and Princess  Ursa , younger sister of  Zuko , and the older half-sister of  Kiyi. She was a key adversary of  Team Avatar, [4]  chasing  Avatar   Aang  and her banished brother far across the  Earth Kingdom  accompanied by her two best friends  Mai  and  Ty Lee.

A firebending prodigy, Azula was sadistic, manipulative, and obsessed with power. She was a skilled strategist who orchestrated the coup over the Earth Kingdom capital Ba Sing Se,[5] [6]  and halted the invasion on the Day of Black Sun.[7]

Azula harbored deep mental instabilities, believing her mother loved Zuko more than her.[8]  Raised by her father in an environment without a mother-figure, Azula had to be nothing less than perfect in her father's eyes just to earn any affection from him. After the betrayal of her two closest friends Mai and Ty Lee, these instabilities were brought up to the surface.[1]  Upon the arrival of Sozin's Comet, Azula was soon to be crowned Fire Lord;[9]  however, her defeat at the hands of her brother Zuko and Katara caused her to suffer a complete mental breakdown.[10]

Following her emotional breakdown, she was put in a mental health institution to recover, though eventually joined Team Avatar on their search for Ursa, as part of her secret agenda to betray them and usurp the throne from Zuko.

Trivia

 * In Toonwriter's video series, Azula's relationship with Bowser Jr. is non-canon.