Saijaku Muhai no Bahamut

By mistake entering a female dormitory's bathing area, Lux, a former prince of the Arcadia empire that was overthrown by a rebellion five years previously, finds the kingdom's new princess Lisesharte naked and provokes her wrath. Lux is then challenged by Lisesharte to a Drag-Ride battle. Ancient armored mechanical weapons called Drag-Rides have been discovered in ruins all over the world. Lux used to be renowned as the strongest Drag-Knight, but because he will never attack in battle, he is now referred to as the "undefeated weakest" Drag-Knight. Lux eventually enrolls at the female-only academy that prepares royals to become Drag-Knights after his duel with Lisesharte.