Imaginary friends

Imaginary Friends is a feature included in The Sims 3: Generations.[1] The Imaginary Friend is a character that a child creates after ordering a specific type of mail-order doll.

After giving the doll a name, the child can summon an imaginary, child-sized version of the doll to be a playmate who will happily hop onto the opposite end of a seesaw. An Imaginary Friend may even run minor errands for the child if they have a decent enough social relationship with it. Only the Sim that made an Imaginary Friend can see it. For example. if the Sim who made the Imaginary Friend hugs it, and a another Sim sees this interaction, that Sim will think the kid is hugging the air. Interactions with an Imaginary Friend can only be done on the house of origin. Once the kid leaves the house’s property, the friend becomes a doll in his or her inventory.

Playing with the Imaginary Friend fills up the social motive in the same way as socializing with real people would. Even as children grow into teenagers, they can keep their Imaginary Friends around, and can even develop a romantic interest in their make-believe buddies. Sims can even marry the Imaginary Friends and have imaginary kids, though a strong relationship must be built, just as with normal Sims. If the Sim is good with the Chemistry Set, they can create a potion that will turn their Imaginary Friend into a real Sim

http://sims.wikia.com/wiki/Imaginary_Friend