Huey Emmerich was born without the use of his legs due to a birth defect and was confined to a wheelchair all his life. As he grew up, he became fascinated with finding ways to replace those legs, which soon led him into the field of robotics. In Snake Eater, Sigint briefly refers to Huey's father as a crackpot in an optional codec conversation with Snake because he says walking tank could never realistically be constructed, despite what Granin says at the research facility.
Huey mentions in a tape that as a boy, he never really thought of the implications of nuclear war until he met a Japanese classmate who showed him pictures of Nagasaki and Hiroshima after the US dropped an atomic bomb on each city. Since that day, he tried to think of a way to deter future generations from engaging in nuclear war. One day, he found a benefactor for his research in the form of former CIA Director Hot Coldman. Coldman took him and advanced AI researcher "Dr. Strangelove" to Costa Rica to begin work on the Peace Walker project, and while working on it Huey developed various AI-controlled machines with Strangelove. He soon fell in love with her, but Huey was not sociable enough to make any moves on her.
Soon after Big Boss and MSF landed in Costa Rica, Huey discovered that Coldman was planning to put nuclear missiles inside Peace Walker, but Coldman responded that the only way to deter a nuclear strike from the Soviets was to show them that the AI-controlled Peace Walker would destroy them where a normal human would back down. Coldman then pushed Huey down a flight of stairs, but he was saved by Big Boss at the last moment.
From that point on, Huey helped Big Boss and MSF to destroy his own creations, including Peace Walker. He also mentions in a tape that one of his favorite movies was 2001: A Space Odyssey, especially its portrayal of the computer HAL (it's inferred that this is the inspiration for Otacon's real name).
After Peace Walker, Huey continued assisting MSF and maintaining Metal Gear ZEKE with Strangelove, even having a child with her.
As the UN and US demanded assurance that MSF was not becoming its own nuclear-armed state, Huey rescinded Kazuhira Miller's refusal to allow IAEA inspectors aboard Mother Base. Huey claimed that allowing inspectors to tour the base and find nothing wrong would boost MSF's reputation to the world.
However, the inspection turned out to be a ruse, as the secretive XOF unit set up the inspection to destroy Mother Base from the inside. Huey was kidnapped by the unit's leader, Skull Face, and was set to work on creating robotic weapons for the Soviets.
The Phantom Pain
Over the next nine years, Huey managed to create robotic legs that would allow him to walk, and used similar mechanics to construct the miniature Walker Gears for the Soviets in Afghanistan. On top of that, he constructed a prototype Metal Gear called the Sahelanthropus.
Halfway through his last project, he soon grew concerned about his newest benefactor and quietly sent a message to the US that he wished to defect. The message was received by the Diamond Dogs and Big Boss went to Kabul to find him, but Skull Face discovered the message and threatened to kill him. Big Boss followed them back to the Soviet military base and rescued Huey as Skull Face set a functional Sahelanthropus on them, but Big Boss was able to shoot it down with his chopper and escape with Huey.
Huey was brought back to the new Mother Base and tortured by Ocelot and Miller, who were both convinced that he sold them out to Cipher for his own gain. He did not give them the answer they were looking for, but did mention that Cipher was seeking "a weapon to surpass Metal Gear" in Central Africa. The Diamond Dogs expanded their operations to Central Africa while Huey was kept locked in a separate lab on Mother Base, making Walker Gears for DD instead.
After the events that followed, Huey was tortured again until he revealed the existence of a secret Soviet weapons base in Afghanistan called "OKB Zero," currently occupied by XOF forces, that were rebuilding the damaged Sahelanthropus. The Diamond Dogs mustered everyone they could to destroy it, then carried the wreckage back to Mother Base on Big Boss's wishes.
Despite being ordered not to touch it, Huey used the child soldiers that Big Boss rescued to quietly reconstruct the Sahelanthropus. Huey's actions soon caught up with him when the child soldiers used the rebuilt Sahelanthropus to escape from Mother Base. Afterwords, Miller and Big Boss revealed a sealed AI pod from Huey's old Soviet lab that revealed how Huey had trapped Dr. Strangelove inside after she objected to his using their child, Hal, for his experiments. Eventually, Dr. Strangelove suffocated to death.
The Diamond Dogs and Miller demanded Huey's execution during a show trial at Mother Base, but Big Boss instead ordered that he be sent into exile on a zodiac raft, saying that the weight of his guilt would eventually do him in.
Huey isn't mentioned again until Sons of Liberty, where Otacon mentions that Huey's wife (his stepmother, not Strangelove) had an incestuous affair with him, causing Huey to drown himself (and almost taking Emma with him) in despair in their pool.
In Peace Walker, after Big Boss defeats the Pupa, Huey is sent back to Mother Base. If he's placed in the R&D Team he can use his unique Bipedal Weapons Design skill to develop Metal Gear ZEKE. He also provides valuable information how to salvage different pieces and AI boards necessary to develop ZEKE from Pupa, Chrysalis, Cocoon and Peace Walker, along with their Type-II and Custom counterparts.
Hideyuki Tanaka provided his voice in Japanese, and Christopher Randolph in English. An important note is that these are the same VAs who played the voice of Hal "Otacon" Emmerich elsewhere in the MGS series.