The Sorcerer King of Destruction and the Golem of the Barbarian Queen (Manga), Vol. 1

In a land of swords and sorcery, a ritual is conducted to summon forth the "sorcerer king" who will bring about ruin - but succeeds only in summoning an ordinary human boy from Earth. Stripped of his name and memories but endowed with sudden, enormous magic power, our protagonist has no intention of destroying the world. Fortunately for him, tinkering with his newfound magic results in the creation of a powerful (and beautiful) new partner... and she can keep him safe!