Itsuwari no Oujo wa, Ryuuto no Kuni no Saishou ni Dekiai sareru

Rain was born as the king's illegitimate child. She lived quietly in the royal palace, invisible to everyone. Then one day, a major conflict breaks out at the border, and she is forced to go to the dragonkin country as a false princess—a hostage! Though frightened, she is warmly welcomed by the dragonkin, including Prime Minister Sigurd, in the neighboring country, leaving Rain bewildered. She soon finds herself drawn to Sigurd's sincere kindness, but he is actually the person her father, the king, ordered her to assassinate! (Source: Ichijinsha, translated)