Dr. Hal Emmerich , nicknamed Otacon due to his fondness of anime, first appears in Metal Gear Solid as an ArmsTech employee who designed Metal Gear REX. After being rescued by Snake from the Ninja, he begins to assist him after he realizes that REX is being used by the terrorists to launch a new type of nuclear-warhead. Hideo Kojima states that he created Otacon as his way of glorifying the otaku stereotype by having a character who fights the terrorists with his intellect rather than his brawn.According to Kojima, the original idea of Otacon was to make him "heavier, wearing a cap, and programming while eating a chocolate bar". However, the design Yoji Shinkawa did for the character was a slender one instead.
Otacon may be the closest thing that Snake has to a best friend, as he has helped him out from difficult situations; Not only mission-related, but also from personal issues.
Metal Gear Solid 2: Sons of Liberty
In Metal Gear Solid 2, Otacon and Solid Snake form Philanthropy, an anti-proliferation organization. During the Tanker chapter, Otacon once again supports Snake via the codec and is in charge of saving the player's progress. He later appears in the Plant chapter, having infiltrated the Big Shell facility with Snake to save his stepsister Emma, who is among the hostages. After Emma is killed by Vamp, Otacon transports the surviving hostages to safety and then provides support to Raiden via codec for the remainder of the game. Lead character designer Yoji Shinkawa stated in an interview that Otacon was designed to look somewhat "tougher" than he did in Metal Gear Solid.
Metal Gear Solid 3: Snake Eater
Though Otacon himself does not appear in Metal Gear Solid 3 (the game being set in the 1960s), His grandfather (who was part of the Manhattan Project) is mentioned in a radio call between Naked Snake and Sigint (although Sigint is unable to remember his name correctly) and he appears in a photograph alongside Russian weapons designer Aleksandr Leonovitch Granin.
Otacon appears in Metal Gear Solid 4, still lending his support to Snake. He builds with Sunny (Olga's daughter), a robotic companion to Snake called Metal Gear Mk. II, which he controls remotely. The Mk. II is taken from a robotic character of the same name in Snatcher. He becomes romantically involved with Naomi Hunter through the course of the story. In the coda of MGS4, Otacon tells Snake that he and Sunny will live with Snake for the remaining few months he has left to live to serve as witnesses of his existence.
Otacon is also seen with Solid Snake's crew in Super Smash Bros. Brawl. Although he is not a playable character, he makes an appearance when Snake calls him on Shadow Moses Island, and he will talk when the player other than Snake is Captain Falcon, Diddy Kong, Donkey Kong, Ganondorf, Kirby, Link, Mr. Game & Watch, Pit, R.O.B., Samus, Sonic, or Yoshi. He is also a sticker (in his Metal Gear Solid 2 artwork), which may be used in the Subspace Emissary to increase "Slash" Resistance.