Yuusha-sama ni Ikinari Kyuukon sareta no desu ga

A certain beautiful princess was captured by a demon king, and a hero went out with his party of adventurers to rescue her. When he returned successfully, he asked for marriage as his reward... but not with the princess, with her maidservant, Aria Milford! Aria was as shocked as anyone else, since she quite happily considers herself a minor...