Scar
Zuko. Zuzu. He has Father's Mark, of course, but also that delicious splash of pink scar tissue in the center of his chest. Besides the Mark on his face, the lightning strike directly to his heart couldn't be placed better.
Avatar Aang. A circle of boiled flesh on his back, forever ruining the blue tattoo line running straight over the spine. If the tattoos, those arrows running along his major Qi lines, are the last legacy of his people, then the scar is a reminder of their fate that will live as long as the Avatar does.
The Waterbender, Katara. She has no external marks on her, but she was the one who had to pull both Zuzu and the Avatar back from the brink of death, had to see those wounds in all their fresh glory. Yes, the Waterbender has scars, but she keeps them behind her eyes.
And yet the last of her oldest enemies, Sokka, has no scars. His injuries all healed fully. He never saw his friends fall. He found that the implications about his captured lover were false. He won all his battles.
Over the years, Azula became relieved that Sokka had escaped her unscathed.
END