Glass avatar

The Glass avatars, or glass people (TV: Twice Upon a Time), were translucent humanoids made out of glass.

History
Helen Clay started up the Testimony Foundation, a foundation of taking people from their last moments and making them into glass bodies, in 500000012 on New Earth. (TV: Twice Upon a Time)

When one glass person tried to take Captain Lethbridge-Stewart, there was a timeline error during the process, putting the Captain into the wrong decade. The glass people then took the TARDIS, with the Captain, the First Doctor and the Twelfth Doctor inside. While investigating the Ship, the First and Twelfth Doctors found the glass woman, who showed that they had "found" Bill Potts, as a glass person.

After finding out about Testimony, the Doctor decided to bring the Captain back to the time of his death. After the Captain and the First Doctor's farewells to the Twelfth Doctor, Bill showed the Doctor herself, but as Clara Oswald, making the Doctor regain his memories of her, (TV: Hell Bent) Then, another glass person as Nardole walked up to the Doctor. The Doctor gave Bill and Nardole one last hug before they teleported away. (TV: Twice Upon a Time)

Appearance
The avatars looked like slender see-through humanoids. They talk and walk like humans. Some avatars can appear as anyone or anything they want, such as Bill, Nardole, and Clara

Behind the scenes
Helen Clay is credited as the Glass woman, even though she is played as a human