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Flora had been contacted by Kireina some time ago, and she had been preparing since then. Kireina decided to attack Hel already and not give her any more time to grow stronger, and Flora couldn't agree more with her decision.
Although she sometimes thought that Kireina was completely insane in the decisions she took, there are also other times when she thought she was on the right… there were many things she often thought badly of her, but she was already an ally of her, and wouldn't simply stop to be her ally now that things had ended as they were.
Check "The Protagonist's Sister Is Actually the Strongest"!