The Guardians of Nature - The Ancestral Warriors

Letícia, the princess of the Winter kingdom, will marry Prince Ryuuji of the Autumn kingdom due to an old agreement between the kingdoms, but her mother left her a diary informing her that she needed to find a trustworthy knight to protect her. She finds Aleph a knight with exceptional qualities, but he hides his real identity, who is Aleph? Why did the former queen establish a marriage agreement? Why Leticia needs to find a knight.