The Skeleton King and the Substitute Princess - Luna and the Cowardly King

Unable to believe that the person who sent the flowers was the king, Luna journeys to the Land of the Dead in place of the princess. Luna's goal, with the special ability to speak with flowers, is to become a "flower master" who, like her late parents, uses the blessings of flowers to improve people's lives. Undaunted by her harsh circumstances, she continues to work hard, and one day, the path to becoming an apprentice to a "royal flower master" opens up. Little does she know that this leads to a fateful encounter... (Source: Kadokawa, translated)