Killjoy

You may be looking for Daphne S, credited only as "killjoy".

Killjoy was a word generally applied to people who ruined other people's fun or happiness.

On Terra Alpha, the word was a cultural stigma, applied to people who rebelled against Helen A and her Happiness Patrol, being sad and wandering the streets alone rather than being happy and with others. One such killjoy named Daphne S was lured by Silas P, and was executed after failing her Happiness Patrol audition. (TV: The Happiness Patrol)

Tanith referred to the Seventh Doctor as a "po-faced killjoy," (PROSE: Falls the Shadow) and Ace later called herself a killjoy. (AUDIO: Mask of Tragedy)

The Monk called the Twelfth Doctor a killjoy. (PROSE: The Persistence of Memory)