Tecteun was a Shobogan and one of the first inhabitants of Gallifrey to explore other planets. She discovered the Timeless Child on one such planet, and adopted her as her own. She later brought the child back with her to Gallifrey.
When Tecteun discovered the child's ability to regenerate, she experimented on several of their incarnations to discover the secret to this ability and eventually injected herself with the genes which had been spliced from the child. Upon doing so she immediately regenerated into a male incarnation. Tecteun later gave his people living in the Capitol this gene so that they would be able to regenerate. This was the foundation of an elite society; they came to be known as Time Lords.
At some point after the Timeless Child's memories were redacted and the child became the Doctor, Tecteun hid the truth in the Matrix disguised in the story of Brendan, an immortal Irish Garda (policeman). The Master suggested that Tecteun 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)