Glass avatar

The Glass avatars, or Glass people, were humanoid bodies made of glass, controlled by dead human souls.

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

When the Glass Woman tried to take Captain Lethbridge-Stewart, there was a timeline error during the process, putting the Captain into the wrong decade where he meets the First and Twelfth Doctors. The glass avatars then took the Twelfth Doctor's TARDIS, with the Captain, the First and Twelfth Doctors inside. While investigating the Glass avatar's ship, the Twelfth Doctor finds a glass person formed as Bill. After their talk with Bill, the First and Twelfth Doctors find the Glass Woman, who served as the interface for the ship.

After finding out about the 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 have slender see-through humanoid bodies. They talk and walk like humans, and 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