The prince is stupid. I would be very worried if he were to become the next king.
The girl, on the other hand, is rather pitiful in my eyes. Unlike Sei, who has more life experience and seems to be made of tougher stuff, Aira is a normal teenage girl (if I remember it right). She was suddenly thrusted into an unknown world, far from her home, her friends, and her family. She didn’t know who to trust. Unfortunately, she put her trust in the prince. It’s not unimaginable why she would do that, as he practically offered himself up to her.
And the prince is a bad influence really. He is self-centered and egoistic. He did “protect” Aira, but it’s kinda like spoiling your child rotten. He doesn’t realized that the supposed protection he is giving is making things worse.
And now that Sei is shown to the public as the saintess, I can imagine things being worse for Aira. After all the works put into Aira to train her as the saintess, she has yet to shown any capabilities, while Sei, who didn’t require constant tending to, has shown great results. Aira is of course going to be compared to Sei, and Aira will come out worse for sure.