Saijaku Muhai no Bahamut  

Undefeated Bahamut Chronicle, Saijaku Muhai no Shinsou Kiryuu, 最弱無敗の神装機竜《バハムート》

In a kingdom overthrown by rebellion five years ago, Lux, a former prince of Arcadia, accidentally enters a women's dormitory bathing area and sees Lisesharte, the new princess, naked. This enrages her, leading to a challenge for a Drag-Ride duel. Drag-Rides are ancient armored mechanical weapons that have been discovered worldwide. Lux was once known as the strongest Drag-Knight but now goes by "undefeated weakest" due to his refusal to attack in battle. After his duel with Lisesharte, Lux attends an academy for royals training to be Drag-Knights.