Billy Cranston
Billy Cranston is a fictional character in the Power Rangers universe. He was the original Blue Ranger in the series Mighty Morphin Power Rangers. After it, he continued to serve as an advisor to the Power Rangers team until his departure from the series during Power Rangers: Zeo. He was one of the longest-serving Rangers after Thomas Oliver (Tommy), and Adam Park. He was played by David Yost.
Billy was originally the 'brainy' and 'nerdy' member of the team, coming up with ideas and gadgets to help the other Rangers and speaking in a frequently incomprehensible manner; that of technotalk. He developed quite a large fan following among certain watchers, and was intensely disliked by other viewers. This was partly due to his over-characterisation, leading Billy to appear weak and clumsy, in stark contrast to the other members of the team. Over time, this changed (notably in the alternate-universe Mighty Morphin Power Rangers: The Movie, in which Billy is seen performing backflips). Billy's character was fully-developed by the third season, at which point he no longer wore glasses and appeared to be stronger and a more accomplished fighter.
During the Mighty Morphin Alien Rangers miniseries, Billy's role changed. He no longer fought as a Power Ranger, but rather acted solely in an advisory and supporting role. In fact, Billy was the only Ranger in this period to be portrayed by his own actor - the others had been replaced temporarily by child actors after Master Vile had used the Orb of Doom to revert time by approximately ten years. Billy countered this reverse-aging by restoring himself with a special device that utilised the Power Coins. The device was destroyed by Goldar and Rito Revolto along with the Power Coins before it could be used on the others. Billy acted in this period as a liaison between the Aquitian Rangers and Earth. It was the cure for Billy's reverse-aging that eventually served as the plot device by which he was removed from the show.
The writers left the choice open as to whether or not Billy would become a Ranger again, with him musing at the start of Power Rangers: Zeo that he could always take Tanya Sloan's Zeo sub-crystal in an emergency. He was to become the Gold Ranger while the original - Trey of Triforia - returned to Triforia on sabbatical to try to unify his three selves, but the 'negative proton energy' his body had absorbed in the Command Centre's explosion prevented this.
He did not receive any Ranger powers again, and was written out of the series near the end of Power Rangers: Zeo, supposedly because David Yost had asked for more money and/or had a falling out with the Production Crew (Austin St John, Thuy Trang and Walter Emmanuel Jones had been written out of the series for similar reasons in the second season). Some credence is given to this theory by the many annoyed looks he had during the Zeo bloopers played during post-show credits. Billy was last seen in one of the final Zeo episodes suffering from accelerated aging. He had travelled to Aquitar for a cure from their Eternal Falls (analogous to the Fountain of Youth), and opted to remain there with a female Aquitian he had fallen in love with, Cestria.
David Yost had only a small role in Billy's final episode, his last appearance on the monitor being stock footage and his character being voiced by someone else.