>>3785702
Thing is Ralsei has more mannerisms like Toriel than Asriel, and we know Susie holds a high opinion of Toriel.
People have also kind of conveniently forgotten that moment in Chapter 1 where Ralsei reveals his face and Susie freaks out at it. We know now she doesn't know what Asriel looks like, so she's probably freaking out that he looks like Toriel.