Ace (Death Comes to Time)

Ace was a human that travelled with the Seventh Doctor. She later travelled with the Time Lord Casmus and Midas the Cat. She was grief strucken when Casmus died at the hands of Tannis.

The Seventh Doctor later sacrificed himself to kill Tannis and save her.

The Kingmaker suggested that the Doctor may have only died in this universe. (NOTVALID: Death Comes to Time)

Behind the scenes
The Big Finish audio story Zagreus made it even clearer this was an alternate universe.