Murao Mima has created a robot daughter named Key, but after raising her for a very short time, Mima dies, leaving behind cryptic messages telling Key how she can become human. Key must struggle alone to learn the harsh lessons of life and search for the 'key' to her own dream: the power of 30,000 friends to make her a real human girl.
There is a legend of an angel who fell to Earth many years ago, and was forced to marry a man because he held the key to her only way home. Hundreds of years later, sixteen year old Mikage Aya is the reincarnation of Ceres, the vengeful angel, who must now fight for her life against her family and her own twin brother Aki, the reincarnation of her past evil husband...
Having failed to earn admission to a university, Hideki Motosuwa has moved to the big city, determined to study his hardest for next year's exams. However, an unusual distraction presents itself one unsuspecting day in the form of Chii, a robotic young girl that has been discarded in the trash. In a world where an increasing number of people turn to these 'persocoms' for company, the bonds and limits of human relationships are tested as flesh manages to fall in love with the machine itself...
Both series draw from the mystery of the main female lead coming to terms with who they are. Key is more dramatic while Chobits is more comedy.
Ryunosuke and his father are running away from his crazy mother when, feeling the urge to answer nature's call, Ryunosuke finds a cute little alley cat and decides to keep it. Almost immediately after, they are caught in a fire fight with his mother's lackeys. Tragically the poor cat dies and Ryunouske is heartbroken, but luckily his inventor father just happened to bring along a deactivated robot. By placing the cat's brain in the robot, Nuku Nuku is born! Now all they have to do is teach Nuku Nuku how to behave like a normal girl instead of a super-strong, cat-brained robot, while trying to avoid a mother's wrath!
The year is 2179: humans and robots have colonized Mars. A newer Third-Type robot has been designed to interact undetected in human society. That is, until a man named D'anclaude discovers their secret and starts a movement to wipe them out. Armitage is a Third-Type that works for the police with her partner Ross, and now these two must rid the planet of D'anclaude and his evil plans.
The year is 2032. Tokyo has been destroyed by a great earthquake and a new city, MegaTokyo, has risen from the ashes. Humans now live side-by-side with androids known as Boomers who perform many of the menial and laborious tasks that humans despise, but these artificial servants come with a price: they have a tendency to go haywire and attack those they were built to serve. The A.D. Police force was created to try and stop this menace, but its weapons can do little more than annoy the Boomers. Hope lies with the Knight Sabers, four young women with high-tech, armored suits and enough firepower to stop an army - but will it be enough to stop MegaTokyo's greatest threat?