She is the Apprentice to the ExtraOrdinary Wizard 500 years ago, where Septimus meets her when he gets lost in the glass. He gets to know her better in book 5, Syren, when she becomes friends with...
this is a lie
because it has so much salt in it, it is nearly impossible for life to, well, live in it.
yes, there is no such thing as an afterlife. once you die you die.