The Avatar's Protector

It's hard to surprise Luna. No matter how shocking things are, she normally just keeps pushing on. Even when she finds out that she's been stuck in the Avatar State in an Air Temple for a hundred years and that there's a war going on, she adapts to her new life very quickly. But things do surprise and shock her. Hidden in the future, down every possible path, are things she never expected.
There's more to Luna's past and heritage than she knew.

But that's the future, first let's talk about the present:

The master of all four elements, each reincarnation destined to protect the world and its people.

Everyone knows about the avatar.
However, very few know of the Onsra.

The avatar's protector, also the master of all four elements, but destined for something else. To protect the avatar at all costs. The Onsra will live as long as the avatar, and sometimes even a bit longer. The Onsra can be born as close as the same village as the avatar, and as far as a completely different nation. More often than not, the Onsra turns out to be the avatar's soulmate as well as the protector.

Avatar Aang's Onsra is an Air Nomad called Luna, she will journey with Aang, Sokka, and Katara, as well as going on adventures with just her and Aang...

I DO NOT OWN AVATAR: THE LAST AIRBENDER. (But it would be cool if I did)