Atara Kaai was the biological sister of the main Character, Amber. They both attended Camp Kikiwaka together. That was where Atara's life ended: she drowned. She was 13 at the time, two minutes Amber's junior. Her pet owl, Owletta, also died.
Atara was born with a misspelled name: it was originally supposed to be Katara Kaai, not Atara! Atara stuck, and so she was named. She was born two minutes after Amber, and when they were a day old, both of them, they lost their parents. They lived on the street until they were five before the Sterlings adopted them. On her way to visit Camp Champion some years later, Atara's boat flipped and she drowned.