Bocchiman (ぼっちマン) is a manga series written and illustrated by Sakakibara Sousou.
A hikikomori gets texted 'help me' from an anonymous source. Feelings of heroism coax him to the outside world which and he finds has been badly damaged, with broken houses and bits of debris everywhere. He searches for survivors but all he finds are corpses and ghosts till he gets to his sister's body.The guy has esp so he converses with her ghost and she tags along in his attempt to find more survivors. Eventually, they do find someone that casts a shadow, but they may not be as human as they seem.