Shown throughout the series is the fact that Aiko is an android with an outdated computer and poor size memory. As the other three are based on storybook monsters, it's probable that Aiko is an updated version of Frankenstein's Monster. -ko is a common suffix for female names in Japanese, and the "ai" may be derived from "Artificial Intelligence". She performs housekeeping duties for the other girls because of her devoted mind. Her weight is 300kg or more, and she hopes to get her own human-type body.

Source:Wikipedia