Masca (1997)

Blurb:
Asarella is the clumsy apprentice of the great white mage Elywho. Elywho cares for her, perhaps even more than she is aware, but things become more complicated as the prince of darkness falls in love with her. A sexy goth fantasy in the tradition of Neil Gaiman’s Sandman. 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.

Source: www.anisearch.com/manga/3983