The Last Whistle

Twenty-year-old Kaito Rose, who goes by "Ice," never believed in the supernatural until he meets a ghost named Nia by a creek at sunset. His unique ability to see spirits, particularly demons known as Echos, leads to tragedy when his entire family is murdered by one of these entities.
Guided by Nia, a mysterious "Lost Echo" hunted by her own kind, Ice ventures into the land of the dead seeking revenge. After consuming an Echo's lungs to gain supernatural powers, he confronts the God of Death, known as The Siren, who reveals they've been manipulating events all along. The Siren captures Nia and subjects Ice to psychological torture, forcing him to relive his worst memories through a mysterious whistle.
As Ice struggles between the world of the living and the dead, he must uncover the truth about Nia's nature, The Siren's ultimate plans, and his own destiny as someone who can bridge both realms.