It isn’t that she is forgettable, but rather it is me who has read too many stories. I wouldn’t dare to say I know everything about her after looking at her once, but I can most certainly say that I have witnessed stories where characters similar to her are present.
Given enough stories, characters from multiple stories begin to overlap with each other, causing me to forget the details of each character, and leaving me behind with their general traits.
It’s like… if a young female character has the eyes of a yandere, smiles like a yandere, and looks down at the camera while holding a knife, then -without knowing anything else about said character- you would probably assume that she is a yandere. Even though you have absolutely no proof she is one. (She might be just some student who loves cooking, and is about to chop the vegetables.)
The “problem” then becomes: “I have already seen so many characters with similar traits that I start to forget where characters with these traits come from. How can I be certain I won’t forget this new character?”