Kamado Tanjiro is the protagonist of 'Demon Slayer'. As mentioned above, his family was killed and his little sister was turned into a demon.
Tanjiro vows to find a cure to turn her into a human again and wishes to defeat the demon King to save people from suffering the fate of his family and he begins his journey from there.
In his journey to find a cure for his sister, Tanjiro becomes a swordsman and learns water-breathing style from Urokodaki Sakonji, a former member of the Hashira.
Before becoming a demon slayer, Tanjiro used to be a coal burner. To sustain himself and his family, Tanjiro used to go to the woods every day to cut wood. His family consisted of his mother and five younger siblings.
One day, Tanjiro went to the forest to collect wood and when he returned, he found that his entire family was massacred. Only Nezuko was alive and she was turned into a demon.
The person who killed Tanjiro's family and turned his sister into a demon was Kibutsuji Muzan. Muzan is the demon king and the oldest demon.
He is the only person with the power to turn and create further demons. Muzan has 12 moons which are divided into two parts, the upper moons and the lower moons and each of the divisions has six demons.
Tanjiro vowed to defeat the demon king after he learned about him. He wished to do that for revenge for his family as well as his care for other people. He didn't want the misfortune that befell his family, to befall anyone else.