Without Fred, George didn't want to be George anymore, so he told the frog his name was Harry. He didn't look much like his old self, really - hair gone black and grey with magical residue and the remains of a Peruvian darkness powder gone wrong, eyes sunken and tired from witnessing so much hate and violence and death. Besides, they already had a George who worked there.
Of course, that didn't change who he was underneath. Despite the empty feeling in his heart, he fell right back into the whole "creating chaos and mayhem" thing pretty quickly, finding his shtick and going with it to the best of his ability. It was easy enough to transfigure a few fake sticks of dynamite and an enormous, over-the-top plunger from some spare rubber chickens he found lying around. And the explosions he created were just your basic Wildfire Whiz-Bang flung against the stage; they made a terrific sight and didn't cause any real damage. George had had more than his fair share of "fun" products that accidentally hurt someone.
They called him "Crazy Harry" then, half with amusement and half with exasperation. He liked to think that the Boy-Who-Lived would have appreciated that.
And George didn't mind being a running gag; it felt almost like being home at the Burrow again, there in the shabby theatre surrounded by all sorts of weirdos. It was... comfortable. Sam the Eagle was like Percy, all uptight good intentions that ended up going wrong, and Gonzo was his dad, fascinated with things no one understood, least of all himself.
But there was no one like Fred among the small company of pigs and dogs and chickens. There was never anyone else like Fred, not even George himself. They'd put on the front of being identical not just in body but also in mind. But instead they'd been more like two halves of a rather unorthodox whole, like the two headed monster he sometimes saw on the set (the monster's names were Frank and Stein, which made George laugh every time). He and Fred had sometimes gone in different directions, but they had been connected, deep down, and George had always known he could rely on his brother as the push to his pull, the up to his down. Now when he pulled, there was nothing and no one to hold him back. Slowly he learned how to do that for himself.
So George became Harry, Crazy Harry, and he took his pleasure from carrying on the fine Weasley tradition of blowing things up at inappropriate moments. And sometimes, once or twice, when the echoes backstage were just right, he thought he heard Fred's voice, just that tiniest bit rougher than his own, whispering in his ear and egging him on.
I might be crazy, thought George, cackling as he tossed a Whiz-Bang at the feet of a very surprised penguin, but like a wise frog once said, I think it's how I want to be.