After Sei does a number of feats like completely healing injured knights with missing limbs and such using magic. When she uses magic there's a clue that tips off people that she's the Saint. And then purifying a miasma swamp in one of the forests with her Saint power. People began showing great respect towards her as the Saint after this. Then the idiot prince gets in an argument with his fiancee, in public no less and denounces Sei by calling her a fake Saint because he assumes that they made up a fake Saint in order to quell the people's worries.
The king gets involved, the lead court mage confirms Sei is the Saint. Shuts down idiot prince, puts him under house arrest later. And Aira's situation greatly improves after, and since Aira is a gifted otherworlder instead of being the Saint, she aspires to join the mages when she graduates from the Academy.