She is just a guardian.
"I'm just a guardian, Ciel. I can't be with you forever. Because I'm just a guardian, not a lover..."
But she wants the people she protects to be safe forever.
"Sebastian, you will take care of Ciel on my behalf, right?"
She doesn't like betrayal.
"This is the first and last time, Liu! Don't betray Ciel!"
She is just a girl, but she has to bear a heavy fate.