An incarnation of the Doctor who, against her will, worked for the Division on behalf of Gallifrey until she tried to desert her responsibilities, becoming a fugitive. Although both the Judoon and the Thirteenth Doctor's own sonic screwdriver identified her as truly the Doctor, the Thirteenth Doctor was unsure how the Fugitive fit into her biography, as she had no memory of being the Fugitive Doctor, and the Fugitive Doctor in turn claiming to having no memory of being the Thirteenth Doctor, though she speculated she may have existed in the Thirteenth Doctor's distant past, as a regeneration of the Timeless Child.
At any rate, after slipping away from her superior officer Gat, the Fugitive Doctor hid out on Earth using a Chameleon Arch, taking on a human identity as "Ruth Clayton". She had her TARDIS buried near the lighthouse where she stored her true identity and trusted Lee Clayton to protect her, undercover as Ruth's husband. Lee was also tasked with reminding her to "Follow the light and break the glass" when the time was right, as this would lead Ruth to return to the lighthouse, which she was made to recall from childhood, and finally restore her Time Lord memories.