LogFAQs > #932116423

Alright, I get what is happening. I think.
Yasu is Shannon. (thank god it's not Kanon, that would make everything 10 times weirder)
The Shannon that was Yasu's friend is not a real person, but an ideal version of herself, a perfect servant that is loved by all, and what she wanted to become. Now, she no longer wants to become a perfect servant, but a witch instead, so that Shannon disappeared and Beatrice was born. The one that remained is the Shannon we are familiar with, once known as Yasu.

It's ironic that one with Beatrice's blood without knowing it would take up the identity of Beatrice. Shannon didn't seem to be doing this consciously though? Is this what you call multiple personalities or schizophrenia? Either way she has severe mental issues.