Hitojichi Hime ga, Shousoku wo Tatta. Kokurou no Kishi wa Ringoku no Shiitagerareta Hime wo Zenryoku de Aishimasu

Upon his nation's victory against the Kingdom of Sylvario, Brigandian knight Ark McGuine is sent to collect Princess Sonia of Sylvario, whose hand in marriage had been offered in exchange for lesser financial reparations. But when she doesn't arrive by the agreed date, he's forced to spearhead an investigation into why. As a horrible tale of mistreatment and negligence comes to light, he grows more and more drawn to a woman he's never met, but it all ends tragically: Princess Sonia perished on the way to her new home from her country's own capital, and her own family members were at fault! Ark eventually returns to Brigandia, despairing over a love that could have been...until he runs into a girl named "Nia" in his own kingdom's capital who looks exactly like the princess!

(Source: J-Novel Club)