Ouji ni Koishita Minikui Ahiru

Hester, a servant of Princess Triscari, is astonished by a marriage proposal from Crown Prince Alek. He needs to marry in order to inherit the throne after his father's death. But Hester, who grew up as a neglected orphan, could never be the queen! It's not until Alek promises her a chaste marriage and a large donation to a charity does she eventually agree to the proposal. Soon, though, Hester finds herself attracted to Alek's gentle and carefree charm... (Source: Renta!)