Yasu

The true antagonist, known only as Yasu, shortened from the surname Yasuda.

Throughout the entirety of Requiem of the Golden Witch, it is suggested that the name given to Yasu from the Fukuin House is Shannon.

Yasu is the child of Kinzo Ushiromiya and Beatrice Ushiromiya who was born due to the result of incest. Yasu is, at the same time, Kinzo's child and grandchild. As a result, among the three, Yasu is actually the actor who played the part of the "illusory" witch Beatrice during the series. The series hinges on the mysteries surrounding Yasu.