In Metal Gear Solid, Otacon’s real name is Hal Emmerich.
Hal Emmerich is the brilliant engineer responsible for designing Metal Gear REX, the nuclear-capable walking weapon developed at Shadow Moses. Solid Snake initially encounters him inside the REX hangar, where Emmerich is hiding after realizing that his invention can be used for mass destruction.
The nickname “Otacon” comes from “Otaku Convention,” reflecting Emmerich’s enthusiasm for anime and Japanese popular culture. He is not the same person as his grandfather or father, although the Emmerich family’s scientific legacy is important throughout the series. His grandfather worked on the Manhattan Project, while his father, Huey Emmerich, appears in later games.
A common mix-up is confusing Otacon with Donald Anderson, the DARPA Chief, or with Huey. Hal is the younger engineer who becomes Snake’s close ally and later helps form the technological backbone of Philanthropy. His full name also explains why the character is sometimes listed as “Dr. Hal Emmerich” in reference material.