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 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 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 isn't mentioned again in the timeline until Sons of Liberty, where Otacon mentions that Huey's wife (his stepmother) had an incestuous affair with him, causing Huey to drown himself (and almost taking Emma with him) in despair in their pool.
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.