From 2003-2005 I was the in-house copywriter at Wild Planet Entertainment, where I helped develop the toy lines of the Aquapets and Liquifreaks. The best way to describe them is "test tube Tamagotchis". The Aquapets became wildly popular for a few seasons.
The original was a Sega toy licensed from a Japanese trade show. The gameplay was pretty limited, so I gave the critters inside a backstory and characterization with unique names and personalities. The collector appeal was similar to Beanie Babies: character design, clever packaging and a collectors' map. Each came with a little ID card stating its name, origin, preferences and dislikes, and its best friends.
Adding the story details really helped this brand explode in popularity. The Kid Outreach Team reported a little girl holding one up and saying, "This one's afraid of thunderstorms..and so am I!"