Sanemi is Genya's older brother and only living sibling. After a Demon attack befell their family, Sanemi and Genya were the only ones who survived the incident. However, after Genya branded his sibling a "murderer", their relationship began to worsen and decline until the two split and went their own ways. It is revealed during the battle with Kokushibo that despite the whole incident, Sanemi still heavily cared for his younger brother and actually wanted him to live a normal life without the threat of demons. Genya is moved to tears and apologized after hearing Sanemi's reasoning behind his rudeness. After their battle with Kokushibou, Genya apologized again for blaming Sanemi and thanked his brother for protecting him before succumbing to his wounds and crumbling into dust, leaving Sanemi stricken with grief.
At first, Genya treated Tanjiro like an annoying pest and wouldn't even talk to him. This didn't discourage Tanjiro who kept wanting to connect to Genya, pissing him off. However, throughout their fight with Hantengu, Genya started befriending Tanjiro and gains respect for him. Tanjiro also tried to mend Genya's relationship with Sanemi, although his actions earned him a mouthful from the higher-ups. Genya later taught Tanjiro how to do repetitive actions during Gyomei's Pillar training. Tanjiro cries when he hears of Genya's passing.