Goo is a fictional character from the Cartoon Network animated television series Foster's Home for Imaginary Friends, voiced by Grey Griffin.
The fast-talking windbag named Goo Goo Ga Ga (called Goo for short and referred by her full name only in her first appearance "Go Goo Go") has a hyperactive imagination and is always seen wearing her hair in three braids, with a rainbow shirt, overalls, and yellow cowboy boots. Whenever she went near Foster's, she became over-stimulated and created scores of new imaginary friends. She was therefore banned from the house indefinitely.
Unaware of the rule, Mac brought her inside after an incident involving an eight-man toboggan stuck in a tree that Bloo needed (even though it was the middle of June, it was a long story that he didn't feel like telling), which broke the strap on his backpack. In any case, after seeing Bloo, she imagined up a horde of similar versions, all with one extra feature or another. After Bloo let her in the next day, Frankie and Mr. Herriman got the mistaken impression Mac let her in (and that he was in love with her, much to his growing annoyance/anger).
After Goo filled the house with so many friends that it became unlivable, Frankie and Mr. Herriman said that Mac shouldn't come to Foster's anymore, probably ever because they believed Mac was responsible for Goo always coming there. Because of this, Mac finally snapped and let it out on her saying that he doesn't like her and tells her that she's annoying, weird and talks too much and too fast because she's a chatterbox and that everyone thinks/knows she is a nuisance and wants her to just go home. Goo begins to cry as Mac can see her sad face already feeling bad right away, and she drops the box with his backpack in front of him and runs away to the bedroom where Bloo sleeps after Mac yelled at her. However, he apologized shortly afterward for yelling at her and hurting her feelings. Mac saw that she fixed his backpack and told her to stop imagining so many friends to get to know her already created ones. From then on, she learned to control her imagination and was welcome at Foster's. Her name is a reference to the stereotypical sound that a baby makes. Her parents, who don't believe in "stifling her creative mind," allowed her to pick her own name as an infant.
Though she has committed herself to not make any more imaginary friends, she still sometimes does so accidentally or to help with something and has devoted herself to making sure her scores of imagined friends have a place to live. Since her debut, Goo has become a recurring character in the series.
Goo might actually be the most powerful human in Foster's universe. Her extreme and unbridled imagination can allow her to create an imaginary friend to suit her desire, from several friends to help create a paper mashay pony to an out-of-control living car jack. She could practically create an entire army at a whim.