I'm kinda scared to Google what a tulpa is because horror really affects me but I desperately want to know the folklore behind her it's the eternal struggle

A tulpa is not inherently a horror concept, though pop culture has certainly popularized the tulpa=evil connotation! The concept has its roots in Indian and Tibetan Buddhism, and though the way the word is used in modern western spiritualism is really removed from that, it’s not really a horror thing either. You’d probably be safe googling it.

That’s not to confirm or deny that the apparition IS a tulpa. Just that her name is a fun little reference. Lily told her to pick a name “besides what I’ve been calling you,” so she just named off another type of apparition! Hence Lily’s response, “What are you, a pokemon?” to what was a ghost essentially insisting on calling itself “ghost”.