Agnes Nitt
An amateur witch with split personalities that was taught to sing by Nanny Ogg, and who sang with Christine in the Operahouse, often filling in for her from offstage without her knowledge.
Where is she from?
Agnes is a character from the Terry Prachett's 1995 novel Maskerade, the eighteenth book of Diskworld series and the 5th Witches book. For some crazy reason there's never been a live action interpretation of Diskworld, but we do have some adorable fanart:
here's one of Agnes & Christine as they appeared in Maskerade, done by Kate Hughes.
And here's a fun one by Blackbirdink, who has clearly decided to make her look a bit like a particular songstress:
Comic Appearances
GND 166 - Over the phone, Christine tells Sarah to ask Sherlock Holmes if Raoul's friend Arsène Lupin is to be trusted. Meanwhile, Sarah informs her that Agnes Nitt is going to cover for her classes, and that Skeep the Goblin, having seen Satine's movie, is eager to go to Paris as well.
GND 176 - After seeing a mysterious shirtless hot guy taming Bad Horse, the girls see Erik escorting Blind Mag from the building. He got a call from James, and now has Agnes Nitt's notes for Christine. Sarah expects to find Jareth, but instead finds Greebo...