The Extra Boss. Also the Mistress's younger sister.
She is a vampiric magical girl and has been the Scarlet Devil's younger sister for about 495 years.
She's kind of nuts and isn't usually allowed out of the mansion, but she usually doesn't try to let herself out.
She really respects her older sister Remilia, but her raw destructive power far exceeds that of her sister's. Although she is usually docile, her insanity makes it difficult for others to understand her.
When vampires attack humans, they generally try to take their prey alive so that they can suck its blood afterward, but Flandre has always been fed with cooked dishes, so she doesn't know how to properly attack a human. Whenever she tries to attack a human, she can't control herself and blows them away without leaving a spot of blood.
Perhaps she can't tell that her daily meals are made from living humans.
Her meals look like such harmless foodstuffs as cake and tea, after all...