Kusuri no Mamono no Kaiko Riyuu @comic

In this world, monsters consume a person's lifespan in exchange for granting their every wish. However, Nea's mysterious "apothecary" monster seems uninterested in consuming hers.

After wandering into the land of Weem, Nea is offered the illustrious position of Songbearer, and forms a contract with Dino―a monster as beautiful as he is powerful. While Nea adjusts to her role (and Dino's peculiar brand of companionship) she is tasked with the search for the Grimmdoll Chain: an artifact in possession of the elusive Masked Monster. If not her lifespan, what will Dino require of her in exchange for her wishes...?

(Source: Tokyopop)