Otacon, whose real name is Hal Emmerich, is a character from the Metal Gear series who makes his first appearance in Metal Gear Solid. He is the one who helped design Metal Gear REX, believing that it was to be a TMD (theater missile defense) machine that defended the US from possible nuclear attack. He is horrified to learn that he had been duped into helping develop REX into a nuclear weapon of mass destruction and, riddled with guilt, assists Snake in dissembling REX. During the Shadow Moses Incident, he falls in love with FOXHOUND assassin Sniper Wolf, though as she stands in the way of Snake's overall mission, she is killed in battle. If the player submits to Revolver Ocelot's electric torture, he is the one Snake escapes with and will give Snake (and by extension, the player) his stealth camouflage system, seeing no need to hide anymore. Said item is unlocked for use on the player's next playthrough.
He returns in Metal Gear Solid 2: Sons of Liberty as part of the Philanthropy group, an anti-Metal Gear organization dedicated to dismantling the REX derivatives spawning all over the world as a result of Ocelot selling the specs to REX on the black market. His stepsister, Emma (Whom he calls "E.E.") also appears as the chief scientist behind the Patriots' Arsenal Gear project. Otacon oversees Snake's prologue mission on the Discovery tanker, where the US Marine Corps are developing an anti-Metal Gear unit named RAY. The plan is simply to expose RAY's existence to the world, but a siege on the boat by Ocelot, Sergei Gurlukovich and an army of Russian soldiers puts the mission in jeopardy, and the mission as a whole is ruined when a Liquid-possessed Ocelot steals RAY and destroys the tanker. The Patriots also frame Snake and Otacon for terrorist activities, and the two go into hiding until the 2009 Big Shell incident where they assist Raiden covertly in his battle agianst Solidus Snake and his faction. Sadly, Otacon's sister Emma is killed by Dead Cell operative Vamp, though not before uploading a virus that damages GW, a hostile Patriot AI.
He continues to assist Snake in Metal Gear Solid 4: Guns of the Patriots and builds Metal Gear MK. II, a diminutive robotic navigator of sorts to help Snake on the field. The two have also adopted the late Olga Gurlukovich's daughter Sunny, who serves as a computer savant, and the group live aboard a large plane called the Nomad. In the end, with Ocelot foiled and the Patriots finally dismantled, Hal offers to stand by Snake's side in the latter's final days.
Although Otacon obviously does not appear fully in the Big boss-centric prequels Metal Gear Solid: Peace Walker and Metal Gear Solid V: The Phantom Pain, his father Huey appears as a frequent ally of Big Boss and designer of Metal Gear weapons such as ZEKE, Peace Walker and Sahelanthropus. Otacon makes a cameo appearance in Phantom Pain as an infant in a photograph with his mother, Dr. Strangelove. Huey planned to use Hal as a possible test pilot for Sahelanthropus, though Strangelove refused to allow this and was murdered by Huey for her defiance, by trapping her ina n AI Pod and allowing her to suffocate.
Otacon's first name derives from the supercomputer HAL-9000 from the famous sci-fi movie 2001: A Space Odyssey, along with Snake's real name of David. In MGS1's "Otacon" Ending, the two chuckle at the coincidence. His surname "Emmerich" derives from film director Roland Emmerich, and his famous codename is derived from "Otaku Convention," a real life group (albeit spelt "Otakon") that is frequently credited in every Metal Gear game that features the character.
Otacon appears in Shadow Moses Island and communicates Snake via Codec and informs about the fighters in the battlefield. Otacon informs Snake about Donkey Kong, Diddy Kong, Link, Samus, Yoshi, Kirby, Captain Falcon, Mr. Game & Watch, Pit, Ganondorf, Sonic and R.O.B. And, if Snake happens to get K.O'd while Otacon is on the communication line, he says the infamous line no Metal Gear Solid player wants to hear- "Snake? Snake? Snaaaakkkkeee!"