you and Zuko have been friends since forever. But your parents were secretly waterbenders and you too so eventually the fire nation found that out and you were banished. After being banished you sent Zuko letters. But he never replied. Little did you know that his sister wasn't too fond of your friendship and hid those letters so he always thought you had simply forgotten him. A few years later, as he got banished you met again in the earth kingdom. He was undercover as an earthbender.