Who is Number 6 from I Am Number Four and what is their importance?

Number Six is a young girl, who is described to be as strong as John. She is a Garde, who assists John and Sarah in their battle with the Mogadorians. Six is a bit older than John. She has hazel eyes and long dark hair, which she keeps pulled into a ponytail. She is also described as having high cheekbones, olive toned skin, and a wide mouth.
Number Six has come into her Legacy as well. She is able to control the elements, bending them to her will, and she has the power of invisibility like John's grandfather had. Her Cêpan has been dead for three years now, and she has been on her own since. Number 6 tells John that she has been looking for him since she realized Number Three had been killed. She recognizes that they are at war and that time is short.
Number Six stands in battle with John, Henri, Sam, Mark, Sarah and Bernie Kosar as they temporarily defeat the Mogadorians. Though the battle is won, the war is far from over. She knows that they must somehow find Number Five and prepare to make a stand against the many Mogadorians who still inhabit Earth.