Second Tecteun

According to secret information found by the Spy Master in the depths of the Matrix, the Second Tecteun was one of the Founding Fathers of Gallifrey, as well as the first Gallifreyan ever to regenerate.

Biography
After his previous incarnation discovered the Timeless Child's ability to regenerate, she experimented on several of their incarnations to discover the secret to this ability. Eventually she injected herself with the genes which had been spliced from the child, and immediately regenerated. (TV: The Timeless Children)

A new body
As the Shobogans grew in power and technology, Tecteun became one of the leading figures of the new society, based in the newly-built Capitol. He gave the new generation of Capitol-dwellers the regeneration code, but decided to limit the "common man's" regeneration cycle to a maximum of twelve regenerations. Tecteun had laid down the base of the society which would soon rename themselves the Time Lords. (TV: The Timeless Children) most likely becoming Rassilon

Secrets in the Matrix
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. The Spy 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)

Appearance
The Second Tecteun had the appearance of a younger, dark-skinned man with short hair. When he became one of the founding members of Time Lord society, he began to wear a white version of the common Time Lord robes and headdress. (TV: The Timeless Children)