I'm wondering if she's meant to be a stand-in for Mary Morstan. But, for Sherlock, she helps him fake his death. And then she's there for John in the aftermath, maybe even starting a relationship with him, but she knows the truth. She knows Sherlock is alive.

She also knows that when John learns that she knew Sherlock was alive and never told him, he will never ever forgive her. (Oh hey, this turned into a prompt half way though. Oh well, wait for the prompt freeze to be over!)