yeah i realized that -100 corruption is like having 100 corruption in other games. bti weird.
That is by design, this dev works a lot with the deconstruction of themes in her games.
To further explain, like you said, in most corruption games you have characters that start at 0 corruption where they are their normal selves, then stuff happens and their corruption increases to a 100 where they completely lose their identity and become mindless sex addicts. Brave heroines, kind housewives, genius prospects, they get broken and ruined and lose themselves, that's what happens in normal corruption games.
Now, what is "corruption"? Looking at the meaning of the word and how it is used in those types of games, we can define corruption as being something like "a deviation from the true nature", so "corrupting a person" would mean to keep them from their "true" and arguably "best" self. Without trying to get too judgemental about the fetish, doing something like that is kind of mean to the character, a protagonist getting corrupted is essentially a failure, a bad ending to the story.
Now focusing back to Once Ever After, with all this context what would negative corruption be? I have been saying that in most games 0 corruption means a character is their "normal self", but here's the question: Do you think most "normal" people are living completely true to themselves? Probably not, most people live with their own set of contradictions, their own hang-ups they don't fully understand, their own shames they don't fully agree with but can't help but feel. If the corruption value is a spectrum that ranges from "completely true" to "completely misguided", and 0 is just the "normal", then a person needs to go into negative values to get closer to their true selves.
The thing with Ingrid is that she genuinely enjoys sex, but she has concerns about what is considered "proper" and "ladylike", doubts most normal girls have, and her tale needs her to have such concerns because the whole point of her tale is to break her, prey on her doubts and chip away at her sense of self-worth until she just gives up on thinking about what's right or wrong and becomes fully corrupted. If Ingrid already started her tale completely broken or completely sexually liberated then the tale wouldn't work, she needs to be somewhat sexually repressed in the first place for her to be broken. Ingrid going into negative corruption is essentially her confronting her desires and realizing that there is nothing wrong with some of them, they are not intrinsically opposed to who she is as a person and they don't have to overrule every single other aspect of her personality. She can be kinky and she can enjoy sex on her own terms, that doesn't make her a "mindless slut", it only makes her less corrupt, more true to her true self.
