Tomokazuki

On August 7, while Yuza Minakuchi was diving, he encountered a being that looked exactly like himself. Azami wondered if this might be the "Tomokazuki" from a ghost story told among divers.
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In truth, it is actually Yuza Minakuchi's consciousness, split from his immortal body when his head was decapitated during the Tragedy of Kameshima. As a spirit, it serves as the Player Insert Character for the story.
This separate consciousness found a way to Ryugu, a place that lies on the boundary of dimensions. As a spirit, it possessed Yuza and others, then traveled across time and space in attempt to guide the course of the story.