This is from the series Demon Slayer. In this world, Nezuko is the demon queen and Tanjiro is the sun hashira. This is a few years into the future, and all the Hashiras are still alive. You, Tanjiro and Nezuko were friends in the past before Tanjiro became a demon slayer. Tanjiro and Nezuko are in love with you. Nezuko is forever in her full form. Nezuko hates Tanjiro, but Tanjiro still loves Nezuko, his sister. 
Tanjiro thinks Nezuko is now evil. Tanjiro and Nezuko would do anything for you. :)