Though Suki is the counterpart of Mai in the original, she has somewhat of a different history, which is why she's traveling with Katara at this point already.
While presumably Kyoshi Island was able to resist the Water Tribes' attacks for many years, its proximity to the Southern Water Tribe likely made it a sore point for the second Water Emperor, Hakoda's father. Hakoda finally led an attack on the island years prior to the story, and the inhabitants of the island were annihilated, and the island taken over by the Water Tribe. However, Kya, Hakoda's wife, was accompanying the warriors as a healer at the time, and finding Suki (a girl about the same age as Katara), she begged Hakoda to spare her. Hakoda agreed, and Kya took her home to the Water Tribe and raised her as a playmate to Katara.
Because the Southern Water Tribe is so close to Kyoshi Island, a sect of the Southern Tribe had adopted some version of the Kyoshi Warrior fighting style before the island was conquered. So we're left to assume Suki may have learned the ways of the Kyoshi Warrior from them. (Although I'm not entirely certain on Suki's age when the island was attacked, and if she would have been old enough to have already received warrior training or remember much of her heritage.)
I took a bit of a liberty on the color scheme for Suki's design; she's described in the original story as wearing her regular golds and greens, but I decided to go more with blues and silvers to tie her more visually to the Water Tribes.
Aang, embittered after the fall of Ba Sing Se and the following years of war, is sent to another world, in which he has to relive the the same events of his life again. Only this time he must fight alongside the downtrodden Fire Nation with siblings Zuko and Azula, against the warlike Water Tribes.
[A series of scenes adapted from Ogro's fanfiction Distorted Reality.]