Tecteun is a relatively new character to Doctor Who, having debuted in Series 12's "The Timeless Children" back in 2020. Despite this new entry into the show, Tecteun has already proven to be one of the most important characters; having been the person who found the Doctor as a child in the universe and raised her as her own. Some people loved Series 12, some hated it, all because of this revelation.
Fans have been split on the Timeless Child origin story, with a lot of fans also hating Tecteun for effectively treating the Doctor like a lab rat while she was under her care. For some people, she was a one-off character deserving of a return, for others, they never wanted to see her again. Despite this, Tecteun showed up again in Flux, this time played by Barbara Flynn, and reception was much warmer. Tecteun can be considered a major player in Doctor Who lore, so it is important fans know as much as they can about her.
5 She's Otherwise Known As Awsok
When Tecteun returned to Doctor Who in "Once, Upon Time" (Episode 3), she was instead credited as Awsok. Now, behind-the-scenes, this was obviously to prevent the audience from discovering she was Tecteun pre-emptively. However, this is Doctor Who, and Tecteun could always show up in the future using her Awsok moniker.
Either way, the TARDIS Wikia has updated it and clearly made it part of the lore, and fans shouldn't be too surprised if in the future there is more expansion on Tecteun using this other name. Tecteun wouldn't be the only character to have been known for multiple names. Every version of the Doctor has had aliases, such as 'the Oncoming Storm', 'Merlin', and 'Raggedy Man'.
4 She's The First Time Lord
A lot of fans incorrectly assume that the Timeless Child origin story means that the Doctor is the first Time Lord, but this is not the case. Tecteun managed to experiment on the Doctor as a child and replicate the regeneration ability into herself, regenerating her into her second incarnation.
This makes her the first Time Lord, as she was a native to Gallifrey, the home of the Time Lords, whereas the Doctor was from a different universe. If being the adopted mother of the Doctor wasn't enough to propel her to Doctor Who lore importance, Tecteun is literally the core pioneer behind the Time Lords ascending to the heights they did. This puts Tecteun in revered company, as Rassilon and Omega were some of the Classic Series' best villains.
3 She's Probably Younger Than The Doctor
The age of Tecteun has never been explored, neither on-screen nor in any Doctor Who expanded universe. It is possible that she's older than the Doctor, but considering she seems to have stayed largely static; staying on Gallifrey and then as one of the heads of the Division, there's a good chance she's younger.
The Doctor left Gallifrey in a faulty TARDIS back in his first incarnation (William Hartnell) before his Doctor's first episode and has been traveling across time and space ever since. This has led to the Doctor becoming thousands of years old, which even by Time Lord's standards is considered pretty old. Not many people can say they're older than their parents, even adopted, but the Doctor embodies the timey-wimey nature of the show based on them.
2 She Was In The End Of Time (Probably...)
Tecteun wasn't even conceived back when "The End of Time" was released back in 2010, Back in that episode in which the Time Lords, led by Rassilon (Timothy Dalton) return to the universe to continue their quest to destroy the whole universe, fans were interested in a mysterious female figure (Claire Bloom). Russell T. Davies (the writer of the episode and showrunner at the time) publicly stated he wrote it with the idea that the character was the Doctor's mother, even though this was never explicitly stated in the episode.
There's a good chance that this was an incarnation of Tecteun that was for some reason held captive by her fellow founding father Rassilon. If so, this incarnation of Tecteun was certainly more caring towards the Doctor than the ones we've seen during the Chris Chibnall era. To say that she may have been in the Tenth Doctor's final episode, the seeds have been sewn for Tecteun for quite some time.
1 She Is Probably Not 'The Other'
The canonicity of the prose novel Lungbarrow is up for debate, but as it's never been explicitly confirmed nor denied by the events covered in the show, fans are free to consider it as part of their headcanon if they want to. Strictly speaking, fans are free to consider that either way. It's not as if there haven't been weird modern Doctor Who episodes that deal with similar lore-breaking material. The idea of 'the Other' is that it was the third Founding Father of Gallifrey, alongside Rassilon and Omega, who was reborn/ reincarnated via a 'Loom' and became the Doctor.
After Tecteun's existence was revealed, a lot of fans online began wondering if she was the Other. As it stands, it stands to reason that the Other was the Timeless Child, who eventually as we know had their memories wiped and was reborn into the Doctor. Tecteun was simply another Founding Father, who perhaps had her involvement in pioneering Gallifrey's ascension removed from the archives either to keep the Division secret or simply due to arrogance or jealousy from Rassilon and/or Omega.