Greasy Johnson is the village bully of Lower Tadfield in Good Omens. He is large and fat and so clumsy that he hurts other children whenever he plays with them. He apparently chose to be a bully because it gave the illusion of control, he could now say he meant to hurt other children instead of shame-facedly admitting he couldn't help it. In private he collects tropical fish and it is suggested that he was one of the babies switched by the satanic nuns of the Chattering Order of St. Beryl in the opening chapter (possibly he is the American diplomat's natural child).
Greasy's gang, the Johnsonites, are not described. It's safe to assume that they are the kind of sneaky little ferrets who follow a bully around and then stand at his shoulders tittering as he does his bullying. Adam Young considers his gang, the Them, to be the goodies and the Johnsonites the baddies, but he ruefully admits that most adults don't want either gang around.
In the closing chapter, where everything is put right, it is suggested that greasy will get a Happily Ever After too, as he has just heard that a fad for playing American Football is sweeping England (as indeed it did circa 1986-87).