Eve's Heart is a special magical construction found in most machine dolls, providing them with human intelligence. Under some talented machine doll creators, it can also provide the machine doll with the ability to sweat, digest food and various other human-like characteristics. Dolls without the Heart will have to depend on the Puppeteer's own will to move, unlike the machine dolls with the heart, which can generate enough magic to activate their own movement.
Eve's Heart is the only known magical circuit that can override the "Impossiblilty Principle of Magic Stack", allowing the machine doll to use other forms of magic like "Absolute Barrier" (Alice's Shin), "Kongouriki" (Raishin & Yaya's) and "Mist" (Snow White).
A Bandoll is a Machine Doll created with wholly organic parts, i.e. with real bodies, fitted with Eves Heart to animate them. Frey's Garm dogs are such an example, them being failed experimets of D-works to create true Divine Machine Dolls. Taboo Dolls made from human body parts are also known to exist, as shown by the dolls used by Magnus. These dolls are much stronger and agile and can withstand greater amount of magic to amplify their magic circuits than normal machine dolls.
It is known that Eve's Heart can also be implanted into a human body, giving it's user increased strength, agility and regenerative abilities. Examples include Dworks' Loki and Frey, and also Alice. These puppeteers still retain the ability to command machine dolls. However, as experienced by Loki and Frey, they will have to suffer from occasional spasms as a result of the overloading of chaotic magical energies within their bodies until they managed to control their own magical circuit. It is also noted that as a result of the Heart, Loki's blood is extremely potent in magic and can cause the magic saturation in an area in multiply just by spilling his own blood.
The method of construction of a Heart is unknown. However, the ultimate goal of machine doll creators is to use the Heart to create a true human being and this is a subject of intensive research in many countries such as Great Britain and Germany. Examples of their endeavors include Prof. Eliade, an English with her doll (Evansjrin, "Absolute Monarch") and Alice from the Academy (Shin, a Divine Machine Doll with "Absolute Barrier"). These machine dolls are completely independent and do not require their puppeteers to provide them with magic to activate their own magical circuits as they are perfectly capable of generating their own magic, albeit it being a little lesser than what the puppeteer can provide.