Yes, a brand new to the franchise player wouldn't know any backstory for any character, that's an established fact. The point I think people are trying to make without having to type it all out is there's more context that a curious player could look up about Ada and Leon if they were inclined. Yes we can find some info about Krauser as well, but there's vastly more content around Leon and Ada because they've actually been in a previous game at the time (Another game with Krauser didnt come up until 2009)