Alice's mother?

Her mother died when she was born,

That’s not the impression I got. In the scene where Alice storms outside and her father follows, she says, “She doesn’t make you truly happy. Not like Mother.” And also, “It took you years to get over Mother.” Sounds to me like Alice knew her mother and there was a time when she and her parents were a happy little family. She does say that her father was sad for her whole childhood, so I think they lost her mother when Alice was quite young, but still old enough to be able to remember her.

I’ll be surprised if there isn’t a WL related twist to her mother and/or what happened to her. And I think her father knows about it, which is why it stresses him out that Alice has been to WL now too.

Everything Alice knows about her mother was told to her, because she never met her. In episode two she says, “My mother used to play. Or so I was told. She died when I was born. I thought if I learned to play like her it would make my father happy.”

Right you are, I’d forgotten about her saying that. It’s in the scene where Cyrus is teaching her swordplay.

Just because she was told that her mother died when she was born doesn’t necessarily make it true though. If something WL-related took her mother away, her father could have told Alice a lie about it. Like we’ve seen Neal say, it was easier to tell the Darlings that his father was dead, rather than admit that he’d abandoned him. Perhaps Alice’s mother left her father to go to WL, but to protect his daughter, so she wouldn’t grow up feeling abandoned, instead he just told her that her mother died in childbirth.