Lux, a former prince of an empire named Arcadia that was overthrown via a rebellion five years earlier, accidentally trespasses in a female dormitory's bathing area, sees the kingdom's new princess Lisesharte naked, incurring her wrath. Lisesharte then challenges Lux to a Drag-Ride duel. Drag-Rides are ancient armored mechanical weapons that have been excavated from ruins all around the world. Lux used to be called the strongest Drag-Knight, but now he's known as the "undefeated weakest" Drag-Knight because he will absolutely not attack in battle. After his duel with Lisesharte, Lux ends up attending the female-only academy that trains royals to be Drag-Knights.
(Source: AnimeNetwork)

About thirty years prior to the start of Hagure Yuusha no Aesthetica, a number of selected people from Earth were summoned to other worlds with only about half of them being able to return. One of the young people chosen was Akatsuki Ousawa, who was sent to Alayzard to defeat the Dark Lord Garius, which he did, before returning to his original world. Some of the successful chosen ones brought back little trinkets as souvenirs. Akatsuki, on the other hand, went a different route—he brought back the defeated Dark Lord's daughter, Miu, who is now posing as his long lost little sister!

Upon returning, Akatsuki and Miu are forced to join a special school called BABEL where they must train to one day master the powers they gained in Alayzard, in hopes that they will one day be able to serve mankind. Will Akatsuki and Miu be able to keep her true identity a secret or will their plan to live peaceful lives together go up in flames?
(Source MyAnimeList)