The Summer Hikaru Died, Vol. 2 (light novel) by Mio Nukaga, Ajani Oloye, Mokumokuren

Something has replaced Yoshiki’s best friend Hikaru Indou. Yoshiki knows this, but he can’t bear to leave this new Hikaru’s side. As the pair return to their old routines together, Hikaru shows more and more frightening signs that he’s not human, and that’s when Yoshiki accepts that he may have to resort to drastic measures. Meanwhile, a mysterious man named Tanaka has found a lead on the eldritch being he believes is connected to the strange incidents occurring in Kubitachi―and it takes him straight to Yoshiki and Hikaru…