Priestess (Ring Theory)

Among the memories within the Gestalt of which Kady Williams saw glimpses as the Gestalt's host were images of a red-headed, veiled priestess. In some visions, the people around her bowed down to her, while in others, Kady saw the priestess being placed on trial. Kady believed that the priestess may have been the being who would go on to become the Gestalt, although she thought it equally plausible that she was merely one of the Gestalt's earliest hosts. The Gestalt, cagey about their origins, refused to confirm either way. (PROSE: Ring Theory)

Behind the scenes
The Gestalt as it appears in Ring Theory previously appeared in Charles Whitt's own story Perfect Authentic Cadence, released in the fan anthology Shit Trips Volume 2.5. This story made it explicit that the Gestalt was a future form of the Doctor, arising after the "Last Battle of Garsennon" makes them unable to regenerate normally. Hints about this backstory persist in Ring Theory, such as the description of the Gestalt's timeship as having a blue and rectangular exterior. As such, if Kady's supposition that the red-headed priestess was one of the faces of the Gestalt's original identity rather than an early host of the modern Gestalt, this would imply the red-headed priestess was an incarnation of the Doctor.