The Blue Shrine Maiden is only mentioned in dialog between Mei, Kuon Glamred Stroheim and Tōya Kagari. She was born with an innate ability to communicate with the Blue when she came into contact with the Sheol Gate. During one of her talks to the Blue, the Gate began to open and the Black Beast surfaced out of it. Believing that she may have summoned it, she beckoned the Beast back into the Boundary and sunk in the Sheol Gate alongside the Beast; she then sealed the Gate from the inside, sacrificing her life to save millions of others.
From then on, her family, the Amanohokosaka’s, began to guard the land, Wadatsumi, that the Black Beast emerged from. Each of her descendants protected the area for millennia to come, until they eventually began to forget their duty; this was until the events of XBlaze.