Legion is a unique geth mobile platform, designed to operate outside the Perseus Veil and interact with organics directly. To that purpose, it houses 1,183 geth programs, as opposed to the one hundred in other platforms, enabling it to operate independently and speak. Appearance wise, Legion presented an opportunity to develop the basic geth design by adding moving parts that allowed it to animate with detailed expressions. The flaps on Legion's head serve the same purpose as eyebrows to form expression, raising to imply surprise or interest and folding forward much like the furrowing of the brow to show concentration.It was created and dispatched following Commander Shepard's destruction of the Old Machine "Nazara", known to the rest of the galaxy as the Reaper Sovereign, investigating several of the worlds Shepard visited — Eden Prime, Therum, Feros, Noveria, Virmire, Ilos, and a dozen uncharted worlds. Eventually it found the Normandy's crash site on the planet Alchera, salvaging a piece of Shepard's N7 armor and incorporating it into itself to repair damage sustained on Eden Prime after being shot by Alliance soldiers. If questioned about specifically using Shepard's N7 armour to repair itself, Legion becomes evasive, first rationalizing with "there was a hole" and then states "no data available" after being pressed, suggesting that it was the result of an irrational action, which goes against the concept that every action the geth do is the result of calculation, unaffected by emotion.
Legion is first encountered while searching for the IFF on the derelict Reaper. It is surprised to find Commander Shepard (to whom it refers as Shepard-Commander) alive, but assists Shepard's team against the husks roaming the dead ship. Eventually, it is knocked out by a husk while opening the way for them to reach the ship's mass effect core. If recovered, it can be sold to Cerberus for research (as intact geth are hard to come by), or brought aboard the Normandy SR-2 and stored under guard in the ship's AI Core. It can then be reactivated for Shepard to interrogate it.
