Sally Sanban is the future grandchild of Numbuh 3, as seen in Operation: F.U.T.U.R.E. In this alternate future, the villainous Madame Margaret had turned most of the earth's male population into girls. Sally, however, had heard stories from her grandmother about how, long ago, girls and boys used to play together. She is the girl of the dreams because she is like Numbuh 3.
Posing as a boy, she attempted to join the Boys Next Door and help fight Madame Margaret. She was found out, however, when it was discovered she inherited her grandmother's fondness for Rainbow Monkeys. The Boys Next Door were about to dispose of her until an elderly Numbuh 4 intervened.
When Sally gave the Boys Next Door a girlifying rifle to reverse-engineer it, she gained the trust of Numbuh 4. With the rifles now able to turn girls into boys, the Boys Next Door managed to get past Madame Margaret's defenses, allowing Numbuh 4 to use her time travel technology. As soon as Numbuh 4 made it to the past, Sally destroyed the time travel device, keeping Madame Margaret from interfering with his plans. She presumably no longer exists as shown by Operation: I.N.T.E.R.V.I.E.W.S. as Numbuhs 3 and 4 married each other, unless if there was another man she married with, but then broke up with before she married Number 4. Or her lineage has changed.