Kawaguchi Kaiji manga
The Devil's Castle is a dark and somber place, until the good-hearted apprentice ASARELLA stumbles into this abode on a goodwill mission, against the wishes of her master, the Arch-Mage, ELIWHO. Things sour quickly, and soon Asarella is bound in a blood pact with The Devil. Little does she know that the mage harbors a secret love for her, which will only complicate her training to prepare her for the apocalyptic battle to come, and which sets up the most unusual love triangle of any place or time.
From Evil Flowers: Although this was released after the 12 volumes of Masca, it is a prequel to the main story. It shows how Caien became a demon lord, about his relationship with Rakin, The King of Speira, and the love triangle between the 2 with the beautiful Rea of the mascan race. Asarella also apears, but the story seems to be concentrating more on Rakin and Rea.
Did you think Masca: The Tale of the Great Mage of Hessed was over? Masca: the Kings is the sequel of Masca, in which Asarella goes on a journey to save Kainen. However, the first two volumes focus quite a bit on the development between Rea and Rakin (Kainen's beautiful ex(?) and another demon king).
Remember the tale of the little mermaid that fell in love with the dashing human prince? Now, imagine the roles reversed, as a romance blooms between the beautiful Evyione and a dashing young merman. It's a unique twist on timeless tale that is sure to leave you spellbound.