Gertrude no Recipe

Sahara is a normal high-school girl whose life is turned upside down when she meets "Gertrude," a 100-year-old demon who looks like a teenaged boy. Gertrude is a manmade demon, constructed from the parts of various other demons and brought to life through a "recipe" from an ancient spellbook.
Gertrude searches for the recipe in order to learn more about his origin, and wants to destroy the formula so that it can never be repeated. Sahara becomes a resourceful ally in Gertrude's quest, and they are aided by some very comical and not-very-threatening demons.
(Source: CMX)
Note: Includes the one shots My Grandfather (マイグランドファーザー), Ouse (逢瀬), Tameiki no Kin no Iro (ためいきの金の色), included within Volumes 1, 2 and 5 of the manga.