After reminiscing about the past, Celestine fell asleep while putting her head on Rein's shoulder. After that incident, she trained under Rein and was able to control the summoning of that calamitic monstrosity after six months or so.
Actually the calamity that she summoned was not the strongest form of the calamity and was actually a weakened version. With enough time, that calamity would regain more and more powers, so Celestine also needed to train more and more if she wanted to still control it in the future.
That was why she was a perfect match for Mona as both of them had the needed determination and free time to train all day long in order to gain powers as fast as possible.
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