Avatar the Last Airbender Book One: Water
Hikari is the 10 year old adopted daughter of Fire Lord Ozai. I'm sure that if she knew her dad was going to take over the world she would have left but as luck has it, she doesn't know. Hikari can Airbend and Firebend, but her best friend Ryuu persuades her to not tell anyone and keep it a secret.
When her brother Zuko leaves to restore his honor she leaves with him along with Ryuu. When they meet the Avatar, Hikari and Ryuu end up traveling with them by accident. However during these travels Hikari feels that Ryuu is hiding something from her. Something that might just as we'll change her life, for the better or for the good?
Disclaimer: I do not own Avatar the last Airbender but I do own Hikari, Ryuu and their story and the extra plots.