Undefeated Bahamut Chronicle

Lux was once the prince of Arcadia, an empire that fell to rebellion five years ago. While trespassing in the female dormitory baths, he accidentally sees Lisesharte, the kingdom’s new princess, naked-earning her fury. She challenges him to a duel using Drag-Rides, ancient mechanical weapons unearthed from ruins. Though Lux was once known as the strongest "Drag-Knight," he now carries the mocking title of the "undefeated weakest" because he refuses to attack in battle. After their duel, he winds up enrolling at the all-female academy where royal heirs train to become Drag-Knights.