Irma represents the evil and the temptation brought by Nazism.
She is the serpent in Eden
Ingrid represents the devil who in the form of a snake tries Eve to betray his Adam. This could be a metaphor to describe this character who officially does not have a well-defined position in the film, as she is not a German officer, and it is not clear what kind of official relationship she has with Bergman other than to control it.
She seems the demonic figure par excellence, able to offer Marina everything she wants in exchange for her soul. Marina must “only” betray the man she says she loves. Ingrid represents the quintessence of the evil of Nazism but above all represents the temptation to give in to threats rather than face them. Ingrid represents the temptation faced by many Italians during the Nazi occupation.