Ruby Sunday

"Ruby S" was an individual mentioned in the Fifteenth Doctor's diary, although her surname was obscured, detailing the events of his first adventure. (PROSE: )

The Doctor, early on, reflected on all the new things in his life, including his "new friends". (PROSE: )

Behind the scenes

 * Ruby's name was only visible as "Ruby S" in First Day of the Doctor, but other non-fiction included in the annual, and concurrent promotional material, revealed her full name to be Ruby Sunday.
 * A clip of Millie Gibson as Ruby was used in the Whoniverse ident released on 31 October 2023.