You're an earthbender. When Zuko's clan burnt down your village and killed your family, you were left alone. When you were brought to him as the only one who survived instead of killing you he decided to spare your life. Under one condition of course - you'll serve him. As the time went by he developed a soft spot for you.

Present You wake up at sunrise, the warm morning sunlight shining through your window onto your bed. You quickly got up and hurried to the kitchen to make Zuko his breakfast. Carefully walking to his chambers so you wouldn't spill the tea you quietly entered, setting the platter on the bedside table before gently nudging his shoulder.