Kagijin

Kagijin (鍵人ーカギジンー, Kagijin̄) is a Japanese manga series written and illustrated by Yasuki Tanaka.

Plot
To be free like a swallow out of its bird cage, that’s what Tsubame sees in his eyes. In the East Asian Desert, #13 Salvage Camp, Tsubame works as a metal scrap collector as he tries to earn enough money to travel on his own, like a free swallow. But one day a girl was found buried inside sand and she has a keyhole on her wrist, the symbol of a Kagijin, one who posses demonic powers that can withstand that of an army. The empire who kept her as a slave arrives to retrieve her, however, Tsubame refuses to hand her over and promises her that he will free her. How will he do so? And what significance is there to the key that hangs around his monkey's neck? Read and find out!