William "Billy" Cranston is a fictional character in the Power Rangers universe. He is the Blue Ranger in the series Mighty Morphin Power Rangers, and was portrayed by actor David Yost. Billy is the only original Power Ranger to remain for the entire MMPR series, and is the second longest-serving Ranger overall behind Tommy Oliver. Until his departure, he was considered the brains of the Power Rangers team, creating many gadgets with which to solve problems that not even Zordon foresaw, and he even invented the first team's wrist-worn communication devices.