A family is attacked by demons and only two members survive - Tanjiro and his sister Nezuko, who is turning into a demon slowly. Tanjiro sets out to become a demon slayer to avenge his family and cure his sister.
Demon Slayer's main character, Tanjiro's life got turned into a walking nightmare when a demon slaughtered his family and turned his sister, Nezuko, to the dark side. Driven by anger and love for his sibling, Tanjiro joins the Demon Slayer Corps to eradicate the demons and, hopefully, find a way to revert Nezuko back to a human.
Nezuko is Tanjiro's younger sister who was turned into a demon. Her transition into demonhood altered her personality, making her far more fearless and direct than she was as a human. However, she retains some of her previous traits, particularly her desire to protect Tanjiro.
Despite being a bit older than Tanjiro, Zenitsu doesn't generally come across that way since his natural disposition is to flee. Although he doesn't always show it, Zenitsu is far from incapable with a weapon.
Inosuke tends to jump into battle without much though. Although he is very much capable of taking care of himself, Inosuke's habit of overestimating his own abilities tends to drop him in boiling water, particularly since he constantly wants to challenge anyone who could possibly match him in combat.