Shiro Kishidan no Himitsu no Oujo-sama Rakuen no Shito ni Ichirin no Hana wo

The second princess is an apostle of the goddess. In order to hide this truth, Liewe was never allowed to appear in public, so he joined the Knights under a covert identity in order to fulfill his mission as an apostle. However, just before his 16th birthday, a strange ball is held in which anyone who can find the second princess will have the opportunity to propose to her! Liewe wants to remain a knight, so he decides to attend the ball dressed as a knight with Gideon, a fellow knight who is the only one who knows her true identity... But for some reason, the prince of a great nation approaches him! A love fantasy about a princess who hides her identity and joins the Knights.

(Source: Ichijinsha, translated)