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She never liked fredrick. He was just there to keep her safe/company. She even made it clear in the previous chapters about their relationship.
Eizer was her crush/like/love way back in her teenager years, remember the flashbacks?? The masquerade party? And now that she realized the man from her childhood is eizer she is drawn towards him, plus eizer's care and a bit of affection in the current chapters also justifies her likeness for him. So imo I don't think it's sudden. They have had so much tensions right from the beginning.
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I mean she’s grown to like her husband it was a process
╮( ̄▽ ̄)╭ it didn’t happen over night all of a sudden and as the other person said he was her crush that she meet from the masquerade party back when she was in ballet school she just didn’t know who he was but she was catching feeling for him before she figured out that it was him. Also I feel like she did at one point “like” the bodyguard/lover cuz they was definitely very close but once she could no longer trust him it was gone
I'm glad she with her husband but like wasn't she with a other guy and now all of a sudden she likes her husband I'm not sure about that like it doesn't feel real