Disclaimer: I do not own Avatar: The Last Airbener or any of the characters from that series.
The blue-eyed child grew up in the Southern Water Tribe with Hakoda, her father and the chief of the tribe; her mother, who was later murdered in the Fire Nation's raid on their village; her brother, Sokka; and her grandmother, Gran-Gran. Katara discovered she was a waterbender- the last in the Tribe- when she was four years old. Knowing that she had the potential to be a great bender, but only if she had a good teacher, her father and Gran-Gran agreed that the girl should travel to the Northern Water Tribe to learn how to bend. She would stay with some distant relatives during her stay.
She was ten years old when she left her home. For a year, she could only learn the most basic techniques and healing from Yugoda because it was against Northern tradition to teach a female how to be a warrior. Soon after her eleventh birthday, however, Master Pakku relented and agreed to teach Katara.
She mastered all he could teach her by age thirteen.
Katara returned to her home, as she had done only twice during her stay in the North. This time, she stayed for an entire year, reconnecting with Hakoda, Gran-Gran, and Sokka. It was great to be with family again, but they all knew that she could not stay forever. There was still much to learn. And besides, her talents would be wasted in the tundra that they called home.
It was with another tearful good-bye that Katara left for Kyoshi Island. From her initiation at age 14 to graduation at age 17, the waterbender wore the green kimono and face paint of the Kyoshi Warriors. The women were some of the best warriors in the world, and were, without a doubt, the most feared assassins. Not the best known, but the most feared. They made sure that each job was clean and no one lived to tell the tale of the Kyoshi Assassins. Rumors were all that anyone ever saw or heard… unless you were a minute from death yourself.
With the warriors, Katara learned how to raid a ship and defend the homeland. She was skilled in hand-to-hand combat and weapons. Her favorite weapons, besides her bending, were throwing knives. The lone bender also learned how to work in teams with her fellow warriors. This was essential to raid an entire Fire ship and come back alive, let alone victorious.
With the few select warriors who were chosen to also be assassins, the blue-eyed girl learned how to go on partnered and solo missions. She learned stealth, precision, tracking, and under-cover operations. They could kill a man a hundred ways with a throwing knife, and fifty more with their bare hands. And that didn't include any other weapon they learned in training.
Katara, who always loathed killing, even for a good cause, took to the practice of carrying a white rose with her on assignments. She would leave it with the body, as a gesture of remorse. Apparently, after an especially bloody murder, the white petals would be stained red. Gossips had given her the name "Red Rose Assassin", and her friends joked and laughed about it, nicknaming her "Rosie".
By the time she was 17, Katara had successfully completed ten team ship-raids and five assassinations of various Fire Nation military leaders and nobleman. Three of those had been solo. At the ceremony to celebrate the completion of training for the new Kyoshi Warriors and Assassins, Katara felt proud of her friends' accomplishments. But she wondered where Fate would take her next.