Kuro Kishi-sama no Oki ni Mesu Mama: Seiryaku Kekkon no Hazu ga Dekiaisareteimasu

Bianca, the first princess of the Kingdom of Northernbleu, is in dire straits after a neighboring country targets her lands. The only way to save her country is to establish ties with a major power... That's when a proposal arrives from Saintrews, the most powerful country on the continent! Bianca, who has known the second prince, Lucas, since she was a child, accepts marriage as a desperate act of friendship. However, he has grown up to become a Knight Commander feared as ruthless and heartless by other nations. Indeed, no longer the kind-hearted boy she once knew. Stunned by his change, Bianca is prepared for his cold-hearted encounter... But what actually awaits are days of sweet favor, drawing her closer to the boy she once knew, now all grown up...? So begins a love fantasy that starts as a political marriage and grows into the knight commander doting on Bianca! (Source: MangaPlaza)