He used so much of his magic power for the summon that he had to sleep for quite a long time.
She has the potential to be an amazing heroine and I'm rooting for her happiness.
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The difference between a no Tachiichi is that in The Saints Power is Omnipotent the people other than the ashole prince treated Sei with due respect and once she started to prove her usefulness they acknowledged her achievements even if they didnt know she was the true Saintess.