I had a weird thought the other day--what if she's the former queen of Lothric? It doesn't say much about the Lothric queen, other than she disappeared after the birth of Ocelotte. She was also said to be fond of looking after the lonely graves of the Undead, offering her blessings in the form of holy water... Maybe she also went a little nuts, and sort of forgot who she was.
Eh, I guess it's a little far out there, but it's fun to think about nonetheless. Maybe she was just another priestess, considering she sells the Priestess Ring in the Untended Graves version of the shrine, and since her outfit is somewhat similar to Emma's.