Second Tecteun

Tecteun was a Shobogan and one of the first inhabitants of Gallifrey to explore other planets. She discovered the Timeless Child on a barren planet and took her back to home and raised her.

When Tecteun discovered the child's ability to regenerate, she injected herself with the genes which had been spliced from the child and immediately regenerated into a male incarnation. Tecteum later gave her people living in the Capitol this gene so that they would be able to regenerate and become Time Lords.

At some point after the Timeless Child's memories were redacted and the child became the Doctor, Tecteum hid the truth in the Matrix disguised in the story of Brendan, an immortal Irish policeman. suggested that Tecteum hid the truth in this manner as an apology or gift for her child and a way for the Doctor to decode the truth if they so chose. (TV: The Timeless Children)