The Summer Hikaru Died, Vol. 5 by Mokumokuren

"Hikaru's" unexpectedly violent encounter with the mysterious hamster-toting Tanaka only worsens his already weakened condition. Without knowing Tanaka's intentions, the boys must act quickly to uncover the mountain's secrets. Investigating the Indou family's "sin" will be difficult now that Hikaru's dad has passed, but they may be able to get some answers out of his closest friend-if Yoshiki's father is willing to talk...