MAHIRU KOCHO , is member of a wealhty family . Her family got murdered when she was 13 years old . Mahiru and her dearest siblings nearly got killed but thank to the rock pillar , she and her siblings managed to escape . On age 14 / 12 Mahiru and kanae , one of her sister , work as a demon slayers . On that time her youngest sister was still training to be like them , she's slowly copying them . After that incident happens , they become the cheerest person ever .....And then that happen DISCLAIMER : KIMETSU NO YAIBA DOES'NT BELONG TO ME .