When the painted lady was mentioned in the book "The shadow of Kyoshi" it made me think about who it was. I knew from the book that she lived prior to 300 BG (before genocide), so going off of that there is literally only one person we know, and she fits perfectly. Ummi is the painted lady. Think, she lived and died inside of 300 BG, with her fiancé Kuruk being the shortest lived avatar at 33, he would have died at around 345 BG, meaning if she fell in love with him, she would have lived during that time. She was pulled under the water in the spirit oasis by that friggin nightmare fuel child-trauma inducing spirit centipede Koh the face stealer and drowned in the water on the day of her wedding to Avatar Kuruk. I think that she was sent to the spirit world or dragged there by the weird spirit centipede. While she was in the spirit world she had her face stolen by he whose name shall not be mentioned, hence why she paints it. If she went to the spirit world, something special would have happened to make her able to go and do that, right? She was pulled into the spirit oasis, right? I think she pulled a reverse Yue and was touched by the ocean spirit before she drowned, making her part spirit, therefore able to go into both the spirit and mortal realm. Also, reinforcing the last one, if she was born in the WaterTribe, why would her hair be black? Well, as we know, when Yue was touched by Tui her hair turned white, so if Ummi was touched by La wouldn't her hair turn black? And, Yue's name means moon and she turned into the moon spirit. Umi means sea. Also, just compare these pictures if you don't believe me.