Mai is one of Azula's childhood friends. She's nearly emotionless and is always bored. She joins Azula and Ty Lee to capture Prince Zuko and Iroh. She hurls shurikens and daggers with amazing precision and accuracy and generally keeps them concealed under her clothing. She is a master of stealth and light weaponry.
Since her father is a big political figure she was always told to sit still, and not to speak unless spoken to. This makes her bored and impassive. She has also harbored a crush on Prince Zuko since her childhood. Later when Zuko betrays Aang they become boyfriend and girlfriend. On Ember Island they do break up when Zuko gets mad at a guy for talking to her, which results in hm breaking a vase. Later they make up again.
When Zuko betrays the Fire Nation to help Aang again, Mai betrays Azula to save him. She and Azula are about to fight when Ty Lee intervenes. This gets them arrested. After Fire Lord Ozai is defeated and Zuko is crowned the new Fire Lord she is released from prison.