Back in the David Tennant era, Doctor Who technically introduced another Timeless Child - Jenny, the Doctor's daughter. Showrunner Chris Chibnall has rewritten the history of Doctor Who, revealing everything viewers thought they knew about the Doctor was a lie all along. The Doctor is not a Time Lord at all; rather, she is the Timeless Child, a being over a billion years old, who became the base genetic code for the entire Time Lord race.
This transforms the Doctor's history and sense of identity. "[A Time Lord is] a sum of knowledge, a code, a shared history, a shared suffering," David Tennant's Doctor insisted in the episode "The Doctor's Daughter." The Timeless Child retcon strips the Doctor of all that, and turns her into a solitary figure. Her true origin is a mystery; she was found by a Gallifreyan explorer, an outcast child stood beneath a mysterious Boundary to another dimension or universe. As far as the Doctor knows, she is absolutely unique on this plane of existence, more alone than ever before. Oddly enough, though, she is completely wrong. Another Timeless Child was created back during the Tennant era.
In the episode "The Doctor's Daughter," the Doctor and his companions Donna and Martha arrived on a distant world where two races had been locked in a war for generations. Ironically, in truth the conflict had only lasted for seven days, because the humans and the Hath had both been reproducing using advanced technology and had no real sense of how much time had passed. When the humans saw the Doctor, they immediately shoved him into a sort of cloning cubicle and created a new soldier from his DNA - Jenny. "Progenation," the Doctor explained, dumbfounded at meeting his daughter. "Reproduction from a single organism. Means one parent is biological mother and father. You take a sample of diploid cells, split them into haploids, then recombine them in a different arrangement and grow. Very quickly, apparently."
Jenny was created from the Doctor's genetic material, and the Doctor began to believe she really was a Time Lord after he listened to her hearts. At first he was appalled at the "daughter" who had been created without his permission, not least because Jenny was a soldier, but she won him over. Unfortunately, although the Doctor brought to an end to the war between the humans and the Hath, Jenny was shot and apparently died in the Doctor's arms. The Doctor believed she didn't share all the abilities of the Time Lords, notably the power of regeneration, and he and his friends left. The final scenes of "The Doctor's Daughter" revealed he was wrong, with Jenny regenerating and leaving her homeworld to begin a new life as an adventurer. Jenny has since starred in a series of Big Finish audio adventures, which are indeed considered canon.
Jenny believes she is a Time Lord, although in truth she has little idea what that really means. In reality, though, she was created from the Doctor - which means she is actually a Timeless Child. The Doctor is not so alone as she believes, because she has a daughter out there somewhere in time and space, a being who shares her unique heritage. Tragically, though, she has no idea Jenny is even alive.