Dogs cast of main characters all bear scars that are reminiscent about their past, as shown in one artwork done by the creator, Mirow Shiro. However, I will only discuss the one Naoto has for obvious reasons.
Naoto bears an X-shaped scar on her chest. She received it when she was young. Events taken place in her past shows that it was cut into her chest when her parents stood to shield her from the blow of the anonymous murderer, who killed her parents by cutting them into quarters. Naoto, being shielded, received less of the blow, but it was still enough to scar her and, almost kill her before Fuyumine showed up on scene.
Scar