Framley is one of the rats that used to work on the Ratbridge Nautical Laundry. Kipper describes him as "'a nasty piece of work'" (15.52), while Tom describes him as someone they felt "'could turn violent at any time'" (15.53). Kipper adds in a later conversation: "'he's about the biggest, ugliest, laziest rat you have ever seen!'" (16.55). In other words, sure sounds like a winner.
His role in Snatcher's plans is revealed when Willbury and the rest are captured, and Snatcher brings Framley up from where he's being kept. Snatcher has been funneling stolen size into Framley, such that he's become enormous. We're talking elephant-sized here. He's got "door-size ears" and "great, red, dinner plate-size eyes" that "swiveled about madly" (45.3-5). Yikes, right? And according to Tom, "'Framley had always had an unpleasant look about him, but now he just looked evil'" (45.14). Enormous evil rats top our list of things that terrify us, just for the record.
Unfortunately, Framley grows to be too big for his britches. When he takes in one tiny morsel of food after his last resizing, he bursts, and gobs of cheese from his insides cover the town of Ratbridge. So long, Framley. At least it was a quick way to go.