Masahito Karakiri serves as the chief priest of the shrine on the island of Hitogashima and is also the head of the Karikiri family, the family traditionally responsible for overseeing the island's rituals. On the surface, he presents himself as a soft-spoken, approachable individual who tends to ramble at length during conversations and enjoys leisure activities such as playing video games. This seemingly gentle demeanor, however, is a carefully constructed facade. Hizuru Minakata, who is acutely perceptive of language and behavior, instinctively feels a sense of revulsion toward him because she senses that his entire personality—his speech patterns, his mannerisms, his actions—is not his own but an imitation copied from others.

The truth behind Masahito Karakiri is far more sinister. He is not a natural human but one of many younger clones of an ancient figure named Shidehiko, who later becomes known simply as Shide. Over the course of three hundred years, Shide has used the copying ability of the entity called Haine to transfer his own personality and memories into successive clone bodies, allowing himself to be reborn again and again. Masahito Karakiri is merely the current identity that Shide occupies, publicly posing as a descendant of the original Shidehiko. In reality, he is the original architect behind the shadow phenomena plaguing the island. He controls multiple clones of himself from behind the scenes and acts as the primary antagonist of the story.

His motivations stem from an obsessive desire to preserve his own existence and to maintain control over the island's secrets, particularly the power of the shadows and the ability to rewrite events. He manipulates events from the shadows, orchestrating the deaths and replacements of islanders with shadow copies in order to further his centuries-old plan. Despite his human body, he possesses extensive knowledge of the shadows’ abilities and is ruthless in his pursuit of preserving his own continuity across time. His key relationships include his connection to Haine, the shadow entity that enables his reincarnation, and his adversarial stance toward the protagonists Shinpei Ajiro, Ushio Kofune, and those who ally against him. He also maintains a deeply disturbed familial link to the Hishigata family, as the child born from a union between the original Shidehiko and a Hishigata woman was actually a clone of Shidehiko—an event that set his cycle of rebirth into motion.

Throughout the story, Masahito Karakiri undergoes little personal change or redemption, as he remains committed to his goal of endless survival through replacement. His notable abilities include the use of shadow duplicates, the direct command over shadow copies, and the capacity to transfer his consciousness into new clone bodies, effectively granting him a form of immortality. He is the final and most formidable obstacle the protagonists must overcome, standing as the culmination of three centuries of manipulation and betrayal.