@randomfanguy I think the point is both sides are part of who Guri is. Hence Devil Guri being hurt that Seiji was adding -San to her name. It was basically denying half of who Guri is.
@InfiniteVerisimilitude You sure? I thought she said he never calls her just Guri. So he called her Guri the one time and she said she was happy, but to go back to using -San from then on.
EDIT: Oh... I went back over the chapter. I see what happened: The dialogue is poorly worded in that scene, which made me read the opposite meaning out of it. Read the TL note this time which clears up what he intended to mean for that scene.