Last Round Arthurs Vol. 1 (manga)

When Rintarou Magami, a teenage know-it-all, sets his heart on something, he always follows through-even if it involves joining forces with the weakest candidate, Luna Artur, in the upcoming battle to become King Arthur's successor. Well, he might have underestimated the extent of Luna's incompetency... but no one could have guessed she would pawn off her one and only Excalibur for cash!