Sanemi grew up with his parents and siblings, but his father was a violent man who frequently beat his wife and children, eventually dying at the hands of someone who held a grudge against him. In contrast, his petite but hardworking mother tried her best to protect the children from their father's violence. Thanks to her devoted protection, Sanemi and Genya grew up well and vowed to protect their mother and siblings themselves. One day, while Genya and the siblings were waiting for Sanemi and their mother, they were attacked by a demon, and all the siblings except Genya were killed. Sanemi fought and subdued the demon to protect Genya, only to discover that the demon was actually their mother. As the sun rose, she disintegrated, and Sanemi was accused of being a murderer by a traumatized Genya. Making it more tragic, this happened shortly after the day they promised to protect the family together following their father's death. This incident fueled Sanemi's hatred for demons. He began hunting them alone, capturing them and burning them in the morning sun. During this time, he discovered he possessed 'Marechi' blood, which intoxicates demons when they smell it. His survival was due to his natural physical prowess and this unique constitution, though he later reflected on how reckless he had been. He eventually met a Demon Slayer named Masachika Kumeno, who introduced him to a trainer, leading him to pass the Final Selection. Masachika saw his own deceased brother in Sanemi, and the two became very close. However, Masachika died during their battle against Lower Rank One, and Sanemi became a Hashira. At his first Hashira meeting, he spoke rudely to Kagaya Ubuyashiki, accusing him of being a superior who just smiles comfortably while others suffer and die. Kagaya calmly accepted the criticism and revealed that he had tried to wield a sword but couldn't swing it ten times without his blood pressure rising to life-threatening levels. Kagaya then handed Sanemi Masachika's will and proved he remembered the names of every fallen slayer. Sanemi realized Kagaya wasn't who he thought he was. Reading Masachika's will, which stated he viewed Sanemi as a younger brother and wished for him to survive even if his loved ones died, Sanemi wept. Later, when Genya joined the Corps to follow him, Sanemi only said, "A piece of trash like you isn't my brother. Quit the Demon Slayers." Much later, in the Infinity Castle... Genya: Brother... Sanemi: ...You really are a hopeless little brother. Why do you think I went as far as killing our mother to protect you? Tanjiro (in Genya's flashback): About your brother, the Wind Hashira... Genya, even though he's very angry that you joined the Corps, I didn't smell a single scent of hatred from him. So don't be afraid. If you have something to say, just say it. Sanemi-san has always loved you, Genya. Sanemi: You should have started a family somewhere, had children, and lived until you were an old man. Everything I couldn't do for our mother, or our brothers and sisters... you could have made up for that by making your wife and children happy. I would have made sure no demon ever got near you... While he was angry at his brother, he finally confessed that he had been harsh only because he wanted Genya to live a normal, happy life instead of one surrounded by danger. After the battle with Kokushibo, Sanemi screamed in agony as he watched Genya's body crumble due to the side effects of demonization, unable to do anything to save him. He wailed as he watched Genya turn to dust. In the final battle against Muzan, he lost his friend Iguro and became one of the three surviving Hashira alongside Giyu and Tengen.