Hi Corbin! I loved the idea you had and even have a character who would be fantastic in this setting! She's not a bender at all, so she could easily be on either side of the conflict. Let me give you a brief biography for her and you can see if she fits your idea or if I should tweak her a bit.
Etsuni is an unusual name for an Earth Kingdom girl, but Zuang Rock is an unusual Earth Kingdom town. It's situated on the northwest coast of the Earth kingdom, an easy journey from any of the four nations. Every year for generations the town has hosted a gathering of strong benders to showcase their skills. Well, up until recently they have anyway. No bender wants to show up at such a public gathering with all the current threats. But for years the town was a tiny cultural hub with art from every corner of the world.
Etsuni's parents met at one of these exhibitions. Chao Bai-ma won all his fighting competitions, and with his talent and wealthy family he could have any woman he liked. He believed that the strength of bending was determined by the parents, so he chose a gifted earth-bending sculptor to give him a son. Instead, his wife struggled through a very rough birth to give him this weak, blue-eyed daughter. Disappointed, he let his wife name her as she pleased. It only disappointe him more when the girl was given such an odd name.
The girl grew fast, though. She was taller and more slender than the other girls in the town and it earned her a hefty bit of teasing. Poor Etsuni was forced by her father into years of earth-bending classes in the hopes that she would manifest, but even with perfect forms she couldn't move a pebble. To give the girl some confidence, Etsuni's mother encouraged her to learn swordsmanship. She took to it like a duck to water.
After Etsuni's mother died, her father became more distant and demanding. He belittled the young woman for her lack of bending and mocked her swordfighting as a worthless skill. He refused to pay for her training or her equipment, so she started working in a teahouse to pay for her hobby. Then he stopped paying for her at all. Though he was the richest man in the town, Chao refused to feed or clothe her.
One night, a village elder came to Etsuni's father's estate to try and plead for the girl. Etsuni overheard the conversation and her father claiming that she was not his child after all! That was enough for her! Without even saying goodbye or quitting her job, Etsuni packed her things and walked out of the town. There was a whole world to see, maybe she would find someone to teach her and make her stronger than any bender.
So from here she might go off and look for the group that are killing the benders, or she might offer her sword to the Avatar and join up with him. I'm game wither way, and I'm willing to change her around as needed.