Hal Emmerich is Emma Emmerich’s brother in Metal Gear Solid 2: Sons of Liberty.
Hal, better known by his nickname Otacon, is a scientist and engineer who first appears in the original Metal Gear Solid. In Metal Gear Solid 2, Emma is introduced as a computer programmer involved with the Arsenal Gear project, and the story reveals the difficult history between the siblings.
Emma blames Hal for leaving her during their childhood and for his relationship with her stepmother. Their strained reunion is one of the game’s more personal storylines, set against the larger conflict surrounding the Big Shell and the Patriots.
Hal’s name is a reference to HAL 9000 from 2001: A Space Odyssey, while “Otacon” comes from “otaku convention.” Emma is not simply another version of Otacon: she is his younger sister and has her own technical expertise and role in the plot.