The Recipe for Gertrude

The Recipe for Gertrude (ガートルードのレシピ, Gertrude no Recipe) is a 5-volume serialization that ran in LaLa starting in 2001. The manga is written by Nari Kusakawa. The North American serialization is available through CMX Manga (DC Comics).

Plot
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 teen aged boy. Gertrude is a man-made demon, constructed from the parts of various other demons and brought to life through a "recipe" from an ancient spell book.

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.