Oh no, I've just had a horrible thought. ðŸ˜
What if Marianne is the character which dies, except that she doesn't die but is transformed by the crests during the five year timeskip into an enemy we've already been introduced to?
Looking at both Marianne and Kronya/Cronje's portraits side by side, they have several similar facial features (nose, ear and eyes) and similar wavy hair.
A transformation from the crests could potentially account for the difference in eye colour, hair colour and greyer skin tone.
I really hope this doesn't happen as Marianne is one of my favourite characters so far.
ðŸ˜
EDIT: Spoke with Thanibomb who said that Marianne should be playable to the end.