The magician guy said that there needed to be another person to help perform the time magic, so I bet the other person was actually the dad, which is why he keeps having the nightmares that he's having. He probably found out after her death that she really was his true daughter and tried to right his wrong.
Also, I don't think Cosette is his legitimate daughter even though she's also a true spirit user. Not sure what her relation is but really don't think she's his kid.
Okay, so here is my theory:
The magician guy said that there needed to be another person to help perform the time magic, so I bet the other person was actually the dad, which is why he keeps having the nightmares that he's having. He probably found out after her death that she really was his true daughter and tried to right his wrong.
Also, I don't think Cosette is his legitimate daughter even though she's also a true spirit user. Not sure what her relation is but really don't think she's his kid.