Who Made Me a Princess Vol. 6 by Plutus
Spoilers ahead for the earlier books.
Athanasia has returned to the palace, but Claude is still ill, Jeanette is still jealous but afraid to tell them she's related, Lucas tries to teach Athanasia magic, and more complications arise. Whether Claude will recover his memory is crucial. Worse, the mysterious figure -- Claude's not dead after all brother -- acts. (He also reveals some of his background.)