The Recipe for Gertrude is a complete 5 volume serialization that ran in LaLa starting in 2001. The manga is written by Kusakawa Nari. The North American serialization is available through CMX Manga (DC Comics).
From CMX Manga: 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.
Gertrude: Is a man-made demon constructed by Claude. He was assembled using choice parts of demons that Claude stole or found. Since his parts are not his own, he spends life on the run from various demons wanting their respective parts back. Gertrude is looking for the recipe for his own creation in order to prevent further cases like his own, to learn more about himself, and also so that demons will stop trying to track him down.
He becomes more powerful during full moons, but has tended to avoid taking advantage of this as it will make him more violent. During these times, his usually silver hair will turn black due to the "evil aura." He was persuaded to use his power by Sahara during a fight when it seemed they would be unable to defeat Curtis. Previously, Gertrude worried he would lose the only part that's his, his consciousness, if he were to use this ability.
It is hinted by Puppen that Gertrude has been known to make human friends every so often, but that they don't last. Sahara's friendship is the first that lasted this long, and the first case where's he stayed in one place. He falls in love with her.
Gertrude has the ability to use anything he can assert power over by writing "Frailty, thy name is (object's name)" with his own blood; as summarised by Puppen and Marriotte, this abilities is basically telling an object, "You are weak, so you'll do what I'll tell you to do".
Sahara Susugi: Sahara met Gertrude while visiting an abandoned house. She is usually calm and level-headed, and often helpful. She visits the house daily to see her friends Gertrude, Puppen, and Marriotte. When the blank pages of the recipe react to her, Curtis suspects Sahara has some connection to the missing recipe, which she denies. It later turns out the recipe has been grafted onto her bones without her knowledge. Sahara falls in love with Gertrude.
Puppen and Marriotte: During Gertrude's construction, an ear was taken each from Puppen and Marriotte. The story begins with the two's attempt to steal them back. They lost, and now keep Gertrude company living in his house. They have the ability to take human form, or take on the bodies of inanimate objects. They frequently tease Gertrude about his not qualifying to be a proper demon; however, after a little while living with Gertrude himself, they act more like regular humans than they do demons.