When Ryunosuke's pet cat dies in an accident, his inventor father "resurrects" the cat by placing its brain in an android body made in the form of a young girl. The Android, Nukunuku, has the dual purposes of being a companion/sister for Ryunosuke as well as being a bodyguard for his father. Nukunuku's extreme strength and feline agility are regularly needed to protect Ryunosuke and his father from Ryunusuke's mother who seeks to kidnap them both following her divorce from his father. Her motives towards Ryunusuke are simple motherly love, however her genius-inventor-ex-husband she wants to put to work designing weapons of mass destruction for her weapons company.

[Source: Anime News Network]