Useless... hmm, so a friend who has done the least within the Sonicverse? Well, to be fair to Charmy, he delivered the walkie-talkie that lead to Chaotix freeing Eggman from jail. Metal had a far more evil plan than Eggman, so it was probably good that Eggman was freed.
I'm gonna say Cream the Rabbit. Her first game she was kidnapped, in Heroes she went off looking for Cheese's brother. As the Sonic wiki states: "Chocola is often regarded as a filler character for Sonic Heroes, created only to act as an appropriate motive for Cream without requiring a separation between her and Cheese, as Cream relies heavily on Cheese to battle and survive."
So Cream's whole mission was filler. Big's mission was more important than Cream's ever was. Cream has never been a helpful character, she always gets kidnapped or looks for useless shit. Thus, she is USELESS.