Tecteun

According to secret information found by the Spy Master in the depths of the Matrix, Tecteun was a Shobogan and one of the first inhabitants of Gallifrey to explore other planets. After becoming the foster-parent of the Timeless Child, a mysterious being whom the Master believed to have eventually become the Doctor, they extracted the regeneration code from their Child, splicing it into their own body and into other Shobogans'. In the process, Tecteun became one of the Founding Fathers of Gallifrey.

Tecteun's incarnations
Tecteun believed that their eyes remained similar across regenerations. (TV: Survivors of the Flux)

In her original, female incarnation, Tecteun allegedly became the first of her kind — the Shobogans — to explore outer space.

As the Shobogans grew in power and technology, the Second Tecteun became one of the leading figures of the new society, based in the newly-built Capitol. (TV: The Timeless Children)

An incarnation of Tecteun also known as Awsok remained active, surviving in the Division's outpost in the Void, well after the destruction of Gallifrey by. She was killed (seemingly for the last time) shortly after reuniting with the Thirteenth Doctor. (TV: Survivors of the Flux)

Relationship with other Founders
Tecteun bears resemblance to Thremix, who was a Gallifreyan biologist behind regeneration in The Scrolls of Rassilon. However, she also becomes the first Gallifreyan to regenerate, an honor that was Rassilon's in The Legacy of Gallifrey.

In the scene corresponding to the point in The Timeless Children where Tecteun's second incarnation stands alongside two other Time Lords in full high-collared regalia, the Timeless Children script release mentions that "we can assume [the other two] are Rassilon and Omega". This confirms that, as far as Chris Chibnall's authorial intent was concerned, Tecteun is not an alternative name of Rassilon or Omega as had been proposed by fans following the broadcast of Series 12, but rather a third member of a triumvirate of Gallifreyan founders. The "third founder" alongside Rassilon and Omega was already a highly mythologised figure in Doctor Who lore, with many stories referencing a mysterious third founder known as "the Other".

The Cartmel Masterplan was predicated on the idea that the Doctor was the Other, or a reincarnation thereof, an idea later confirmed in the novel Lungbarrow; in some way, the story introduced in The Timeless Children splits this conception of the Other in twain, having the Timeless Child be the forgotten earlier self of the Doctor who existed in the days of Time Lord society's genesis, but having the third forgotten founder be their adoptive parent.