So the thing with the scars is actually more about emotional and symbolic resonance than a straight-up checklist of physical conflicts. Like, Tanjiro's forehead scar started as just a burn from protecting his brother, right? But it evolves, literally changes shape after the Final Selection, and becomes this mark that ties him to the Sun Breathing legacy. To me, that's less about a specific battle he was in and more about the inherited weight he's carrying—the loss of his family and the centuries of history behind the Breath styles. It's a visual manifestation of that burden becoming a part of him.
Then there's Giyu's scar on his leg, which is barely mentioned but speaks volumes. We know from the fanbook it's from the battle that cost him Sabito and nearly his own life. That's a major loss, but the scar isn't in a prominent place. It's hidden, much like Giyu hides his survivor's guilt and grief. The placement matters just as much as the existence of the scar itself.
For Shinobu, the lack of a physical scar from her sister's death is its own kind of mark. Her whole insect breathing, her smile, her poison—they're all behavioral scars from that loss. Meanwhile, Obanai's mouth? That's from his literal captivity as a child, a permanent reminder of a battle for survival he didn't choose to fight. It's pre-series trauma made visible, shaping his entire defensive personality and how he interacts with the world, especially Mitsuri.
So yeah, they represent battles and losses, but often in a way that's inverted or internal. The most visually prominent scar, Tanjiro's, is about a legacy he's gaining; the ones tied to direct, personal losses are either subtle or manifested as something other than a wound.