Aunt Sponge is one of the two main antagonists of James and the Giant Peach, the other being Aunt Spiker. She is one of James's two evil aunts and Aunt Spiker's sister.
After the death of James' parents, she and Spiker had no choice but to take in James and "raise" him. Instead of caring for him, she and Spiker force him to do all the work around the house, they had a tendency to beat him when he disagrees with them or backtalk, and almost always make him go into his room without dinner. She and Sponge never call James by his name, but always refer to him as mean and insulting names such as "you nasty little beast".
Aunt Sponge is a greedy, selfish, and morbidly fat woman, who is equally cruel and repulsive as her sister Spiker, and presumably the younger of the two. Both she and Spiker are vain, each singing praises of their imagined beauty while they are in fact repulsive, but each usually ends up attacking the other's repulsiveness. Dominated by Spiker, Sponge is apparently a bit more dim-witted and gluttonous, thinking of eating the peach while Spiker seizes the moneymaking opportunities it will bring to make them rich. Sponge attempts to save her own life instead of Spiker's when she notices the giant peach rolling towards her. However, they trip up over each other and both meet the same end.
She and Spiker adopted James after his parents were eaten by a rhinoceros and are very abusive and treat him like a slave, forcing him to do hard labor and feeding him nothing but fish heads. Both have a terrible hatred of insects and bugs and kill them in a variety of awful ways, and are thus hated by them as well. It is unknown if Spiker and Sponge are the sisters of James's father or mother. In the book, the illustrations show them to be elderly, so possibly his great-aunts.
Sometime later, she and Spiker discover a peach on a withered old tree, and watch it grow to immense proportions in a matter of seconds. She and Spiker use the giant peach as an attraction, making lots of money as James watches from the house, not allowed to leave or play with the other children. That night, she and Spiker send James outside to pick up the garbage dropped by their customers.
Later, when James escapes on the giant peach, he is followed by her and Spiker all the way to New York, apparently somehow managing to drive their now crushed car under the ocean and reach New York covered in seaweed and crabs, where they try to convince the authorities that James is a liar and that he be released to them. However, James, now no longer afraid of her and Spiker, stands up to them and reveals the truth about their horrible treatment of him. Unable to believe that James dares to stand up to them, she and Spiker furiously attack him with firemen's axes until the sudden reappearance of the insects came to James' rescue. James and the bugs tie her and Spiker up with Miss Spider's thread, and the beat cop has them taken away on a crane.
After the credits, there's a scene showing an arcade-like game which consists of controlling a rhinoceros to attack replica models of her and Spiker.
In the book, when the peach gets loose and rolls, it rolls over her and Spiker, killing them both, and they subsequently do not reappear at the end.
In the 1996 film, they survive and pursue James to New York City in their beat-up car (in the Ice World, James and Miss Spider see a shipwreck with figureheads, having a terrible resemblance of the two aunts). Upon arriving in New York, both are soaked as if they had been driving their car across the seabed. They attempt to kill James but are tied up with Miss Spider's silk and arrested.
Spiker and Sponge in the book
Spiker and Sponge ordering James about
Spiker and Sponge in their lounging clothes
Spiker and Sponge posing for a picture
Spiker (right) and Sponge (left) after they chased James and the peach from England to New York