Gunpei (sometimes Romanized "Gunpey" or "Gumpei") Yokoi started out as a simple designer for a card company that had since spread out to produce toys of other varieties. One day, he saw a passenger on a train fiddling around with a simple LCD calculator. He had a brilliant idea from that - what if, in addition to keeping track of time as an LCD watch, the device could also play a simple game? He pitched the idea to his employer, Nintendo.
The product became the Game & Watch, and Yokoi's star was on the rise. He was moved into working on other electronic games, as Nintendo figured that he'd come up with a few more good ideas along those lines.
Perhaps his greatest breakthrough came when Nintendo decided that they would jump into the console market themselves, after seeing how much money could be made from owning the platform and not just licensing games on it. Yokoi looked at the then-common joysticks used for games and decided this wouldn't do at all. They were prone to snapping and were excessively bulky. His innovation was the flat control pad used by the Nintendo Entertainment System - so popular and prevalent, only specialty controllers like ones made for Guitar Hero can not be said to be evolutions of his basic design.
Because this wasn't apparently enough creative output for a guy like Yokoi, he also came up with a couple of game franchises... Metroid, Kid Icarus, and Fire Emblem were also his idea. He also was one of the earliest creators of puzzle games, with Dr. Mario and Panel de Pon (which has been released under multiple names, including Tetris Attack, Pokémon Puzzle League, and Planet Puzzle League) also to his name.
After this, Yokoi turned his attention back to portable gaming. With technology moving forward like it had, he figured that it was time to update the Game And Watch platform to become a true portable gaming system, with interchangeable games. And thus was born the Game Boy, which is up there with the NES controller in terms of all-time influential developments (many eloquent discussions have occurred regarding which is more significant). Along the way, he also worked on the Mario Land series, which introduced Wario and Daisy to the Mario universe.
Things were going great for Yokoi... until Nintendo pushed one of his creations out the door way too early. The Virtual Boy was also Yokoi's invention, and he never recovered from the hit his reputation took. He was Kicked Upstairs, as Nintendo couldn't fire someone of such stature but needed to do something after the Virtual Boy failed.
Yokoi did not take this well at all; he left Nintendo to work with Bandai on their potential competitor to the Game Boy, the Wonder Swan. While it didn't penetrate the North American market well, it held its ground in Japan and Yokoi started to work on improvements for a later edition of the handheld.
Unfortunately, Yokoi never lived to see the results of that. While slowing up near a traffic accident, another car plowed into his, killing him at the scene. There was some controversy surrounding his death (mostly pertaining to the fact that the driver of the car that hit Yokoi's car was a known member of the Yakuza), although most dismiss such discussion as a Conspiracy Theory. The funny thing is that there are some conflicting accounts of the man's death. Other sources report that it was Yokoi himself who had just been in a minor fender bender and was blindsided by the gangster's car when he got out to survey the damage. Also suspicious is the fact that in both versions, a man by the name of Etsuo Kiso, who worked for Nintendo, was somehow involved in the fatal accident. Regardless, the WonderSwan ended up failing in the marketplace after Yokoi's passing.
For a long time, the failure of the Virtual Boy dogged Yokoi's name amongst gamers who knew who he was, though his successes with portables has since started to redeem his name (not-so-coincidentally rising with the popularity of portable platforms). One of his last projects has subsequently been released as Gunpey for various systems, and Nintendo has gone back to praising the late Yokoi's contributions (while all but throwing the Virtual Boy into Canon Dis Continuity).