Kyou kara Anata no Goei desu: Outaishi Denka no Juunenme no Shuuai

Celeste, assigned by the organization, becomes the bodyguard of Prince Edward, the heir to the Midland Kingdom. As they spend time together, she witnesses Edward's strength and vulnerability, and gradually decides to protect him of her own will. Edward, in turn, trains his body and mind, determined to be strong enough to protect Celeste. "I love only you." As they overcome numerous challenges, the two grow closer. However, the barrier of their difference in status—he, the prince, and she, just a bodyguard—becomes an obstacle they must face...

(Source: Julian Publishing, translated)