Glass avatar

The glass avatars, or glass people, were a creation of the Testimony Foundation in 5,000,000,012 on New Earth to preserve the memories of humans about to die within a glass-like avatar. These avatars then were brought to the Testimony Foundation to recount their lives' testimony for the people of New Earth to learn from the past. With the memories added to the Testimony Foundation, the abducted humans were returned to their point of death without memory of their abduction in order to preserve the timeline.

History
The glass avatars were created, under the Testimony Foundation, by Helen Clay in 5,000,000,012 on New Earth. When Captain Lethbridge-Stewart's testimony was added, a timeline error caused by the refusal of both the First and Twelfth Doctors to regenerate in the same time period, caused the Captain to be returned to the wrong decade. To correct this error, the Testimony Foundation abducted the Doctors and the Captain by taking the Twelfth Doctor's TARDIS to their own ship. Here, the Doctors found the Glass Woman who served as the interface for the ship and the glass avatar of Bill Potts.

Upon finding out about the Testimony Foundation and its purpose, the Twelfth Doctor returned Captain Lethbridge-Stewart to nearly the time of his death. When the Doctor and Bill bid each other farewell, Bill allowed the Doctor to regain his memories of Clara Oswald in order to show that Bill's memories within the glass avatar were the same as the Bill the Doctor knew. The glass avatar of Nardole joined the two, and they all embraced before the glass avatars of Bill and Nardole teleported away. (TV: Twice Upon a Time)

Appearance
The avatars' physical form were transparent, humanoid bodies. Any given avatar was able to take the shape and mind of a human whose memories are stored within the Testimony's system as a whole. Forms taken by them include Helen Clay, Bill Potts, Nardole, and Clara Oswald. (TV: Twice Upon a Time)

Behind the scenes
Helen Clay is credited as the Glass Woman on the official website, even though she plays as a human.