The following contains spoilers for Demon Slayer: Kimetsu No Yaiba Chapter 168, "Never Disappear" by Koyoharu Gotoge, now available in English via Animeranku.
Demon Slayer's Wind Hashira, Sanemi Shinazugawa, is known for his grumpy, standoffish personality and for wearing his scars like trophies. He even goes as far as to wear his shirt open to keep his healed battle wounds on display. This roughed-up appearance adds to his tough exterior, but deep down Sanemi has a life story full of tragedy and loss.
The Wind Hashira's life was far from perfect growing up; after losing his mother and siblings to demons, Sanemi took to life as a demon slayer. The scars that cover his body have all been accrued over many years of fighting in battle, but the one covering his face has a far more traumatic backstory -- and acts as a constant reminder.
How Did Sanemi Get the Scars on His Face?
Sanemi was the eldest of seven children living with his mother and father. His father was abusive toward the Hashira's siblings and mother, and early in his childhood was murdered by someone with a grudge against him. After this, Sanemi and his younger brother Genya -- who fought alongside Tanjiro, Inosuke and Zenitsu in the Final Selection -- took to caring for their mother and other siblings.
However, one day the demon slayer's mother went missing; when Sanemi eventually found her, he realized she had turned into a demon and couldn't stop her from returning home to his siblings. Sanemi's mother ended up slaughtering five of the Shinazugawa children, but just before she could kill Genya, he tackled her outside through a window with a knife in hand. In this fight, Sanemi's mother slashed him twice across the face. In self-defense and to save his younger brother's life, the Hashira fought back and ultimately killed his mother.
The Wind Hashira's Scar Acts as a Reminder of That Day
Despite being so proud of his battle scars in Demon Slayer, Sanemi is reminded of that day whenever he catches a glimpse of his reflection. This was arguably the worst day of his life; one where he lost almost all his siblings, killed his own mother and was permanently changed. While the actions were done out of love and an innate need to protect his little brother and salvage the one good thing in his life, Sanemi believes he committed an unforgivable sin.
Even after he gave everything to protect his brother, Genya pushed him away after he killed their mother. Once he was willing to forgive his older brother and understood he did those things to protect them, Sanemi was already too far gone. He joined the Demon Slayer Corps to hide from Genya -- another reminder of his past -- and even though Sanemi was now pushing him away, Genya followed in his footsteps to find him.
Every time Sanemi sees his own face or even his brother, he's faced with the death of his siblings, his murdered mother, and the younger brother he did everything to save who pushed him away.
Sanemi’s Other Scars Distract From His Face
This is why Sanemi is so proud and willing to show off his other scars. He sees himself as a killer and thinks that's something the world should know before they even speak to him. They act as a philosophical kind of armor, used to distract from the two he desperately doesn't want to think about, so they can't hurt him anymore. Likewise, they keep people at arm's length if his putrid personality fails to do so.
After leading a life like his, Sanemi probably wants to keep people as far away as possible. Anyone who gets close to him dies, so the Wind Hashira's rudeness is used to keep him from finding intimacy with anyone. Self-loathing is a huge part of his character, and it goes marrow-deep because how could he not hate himself for killing his mother -- even if it was self-defense. She was the only good part of his life, and Sanemi himself took that away. Even though he devotes his life to protecting people and saving their lives, he believes he will be going to hell; the scars on his face are nothing but a reminder of his trauma.
Sanemi's characterization in Demon Slayer is gut-wrenchingly tragic. From his failing relationship with Genya to the death of his family, the Wind Hashira can't seem to escape loss and pain. Most of his scars protect him from memories of his past and while he wears them with pride, the reason Sanemi does so is truly heartbreaking.