The Saint's Magic Power is Omnipotent (Light Novel) Vol. 6

Sei's life has just returned to a semblance of normal when she's once more summoned to the palace. A foreign prince is coming to the kingdom, ostensibly to study abroad, but his true intentions are shrouded in mystery. In order to protect Sei, the kingdom instructs her to hide her true identity as the Saint and retreat to the safety of the palace. While she complies at first, she soon finds herself drawn to the prince--especially when it becomes clear that she might be the only person who can help him.