Hmm...okay, so they sorta have to deduce their own actions when it comes to this magic. That's a cool mechanic.
This is making some assumptions about how the magic works, but if I had to guess, the other person is her "father".
After she died, the crisis happened and it was revealed that Cosette was NOT the actual Saintess and the stupid-ass duke had the real one executed. The crisis was terrible (and it was all his fault), and in a desperate last resort, they tried to rewind time to fix this.
What a pity dear old dad doesn't get to remember in detail what a colossal fuck-up he's been.