Demyx doesn't wear a scar on his chest to remind him like the others—his scar is on his back. Not that he turned away and ran like some of the others think—maybe a little bit of a flake, perhaps a bit lazy, but Myde wasn't a coward.
No, he went down fighting.
Thin white scars trace across his fingers at angles, barely noticeable compared to the thick pink scars that mar Axel's pale hands and arms. Still, sometimes he looks at the scars on his hands and can almost feel the thin lines of blood as the sitar strings cut him, his hands gripping the neck of the instrument to bring it down like a club or a cudgel on the monster attacking one of his sisters (maybe Lila or Sunila, he doesn't remember, the faces blur together in memory and all that's sure are the screams).
He remembers frail pink light as a heart was released, remembers the light bathing the death of his sister and the birth of a new Heartless, painting his face as it twisted in surprise and then agony, the weight of a Heartless thumping onto his back and then claws digging in to steal his heart from behind. He remembers dying to the sound of his sisters screaming as his world burned down around them, shadows claiming everything.
He doesn't wear a scar on his chest to remind him like the others, but the barely-there scars on his hands are enough to remind him.
Demyx learns how to play his sitar with his gloves on, and tries very hard to forget.