The following post is intended to be a reference about what backgrounds are displayed during the spellcards which have two characters in Violet Detector.
Nightmare Sunday
Remilia, Flandre - Remilia
Byakuren, Remilia - Remilia
Byakuren, Miko - Byakuren
Byakuren, Flandre - Byakuren
Flandre, Miko - Miko
Remilia, Miko - Miko
As you can see, Flandre doesn’t use her background in any stage, but it isn’t present in the game files either.
Nightmare Monday
Yuyuko, Eiki - Yuyuko
Yuyuko, Kanako - Yuyuko
Yuyuko, Suwako - Yuyuko
Kanako, Suwako - Kanako
Eiki, Kanako - Kanako
Eiki, Suwako - Kanako
Eiki and Suwako also don’t use their backgrounds in any stage and the images aren’t present in the game files.
Nightmare Tuesday
Eirin, Kaguya - Eirin
Eirin, Tenshi - Eirin
Eirin, Shinmyoumaru - Eirin
Tenshi, Shinmyoumaru - Shinmyoumaru
Kaguya, Shinmyoumaru - Shinmyoumaru
Kaguya, Tenshi - Kaguya
Tenshi doesn’t use her background anywhere, but it can be found in the game files.
Nightmare Wednesday
Satori, Utsuho - Satori
Satori, Koishi - Satori
Utsuho, Koishi - Satori
Ran, Koishi - Satori
Ran, Satori - Satori
Ran, Utsuho - Utsuho
Ran and Koishi don’t use their backgrounds in any stage and the images aren’t present in the game files either.
Nightmare Thursday
Mamizou, Nue - Mamizou
Mamizou, Raiko - Mamizou
Iku, Mamizou - Mamizou
Iku, Raiko - Raiko
Iku, Nue - Nue
Nue, Raiko - Nue
Iku doesn’t use her background in any stage, but you can find it in the game files.
Nightmare Friday
Suika, Mokou - Suika
Suika, Junko - Suika
Suika, Hecatia - Suika
Junko, Hecatia - Junko
Mokou, Junko - Junko
Mokou, Hecatia - Hecatia
Mokou doesn’t seem to use her background either, but it is present in the game files.
Nightmare Saturday
Okina, Yukari - Okina
Reimu, Okina - Okina
Marisa, Okina - Okina
Reimu, Marisa - Reimu
Reimu, Yukari - Reimu
Marisa, Yukari - Marisa
Nightmare Diary
Yukari, Sumireko - Doremy
Last but not least, Yukari also has her background present in the game files, but she doesn’t use it anywhere during the game.
Below are screenshots from the extracted folders which contain the unused backgrounds. Side note: yes, Mokou’s phoenix can also be found in the files, but she doesn’t use it.