Legion
Role:
Legion is a geth ambassador-platform whose understanding of organics (through their adventures with Shepard) grant them special authority in ME3. They are the voice for the “true geth”. Legion helps Shepard create an understanding between the Citadel Council and the geth, providing the war effort with an invaluable fleet of up to 30,000 ships -- giving the Citadel the resources it needs to combat the Reapers effectively. Legion can also help David Archer (if Shepard completed Overlord) to upload his thoughts and memories to the geth Dyson Sphere. This grants all geth the ability to sense what it is like to be an organic mind – and unites the Heretic-geth with the true geth. Conversely, Shepard can use David Archer to indoctrinate all geth into mental servitude for either the quarians' or humanity's uses.
Recruitment:
Legion is discovered on the lead geth dreadnought over planet Rannoch. Shepard flies to the dreadnought with an intent to discuss the offering of peace that the geth have presented to the suspicious Admiralty Board. Assuming Legion was recruited in ME2, the dreadnought does not fire upon the Normandy, and opens up a shuttle bay so that Shepard's shuttle can enter.
Shepard and company find themselves in gravity-less corridors, with geth platforms lining the walls. They make their way to the main server room of the ship, and find that the room is filled with geth servers (each containing millions of geth) being guarded by geth primes. The geth primes lead Shepard into the ship's main control center, and there Shepard finds Legion. Legion greets Shepard, and explains that, shortly after the commander was taken into Alliance custody they returned to the geth to assist them in preparing for the Reapers. Unfortunately, before they could adequately prepare however, the geth became caught up in another conflict:
If Shepard completed Legion's loyalty mission in ME2, and chose to destroy the heretics, the geth were assaulted by smaller geth-heretic fleets and the quarian flotilla. They retreated to Rannoch, thinking that the quarians wouldn't wish to engage them in combat, so near to their home world (for fear of damaging the atmosphere and planet surface), but still face attacks from the heretics.
If Shepard completed Legion's loyalty mission in ME2, and chose to control the heretics, the true geth's numbers swelled, but were assaulted by the quarian fleet. They retreated to Rannoch, thinking that the quarians wouldn't wish to engage them in combat, so near to their home world (for fear of damaging the atmosphere and planet surface).
According to Legion, peace is still the optimal choice for the geth-quarian conflict. The geth have no real need for Rannoch, and their current stations surrounding the planet are to prevent further assaults from the Reapers/heretics. The surface of Rannoch has been ravaged by husk/heretic camps, and several good sites for quarian re-habitation are currently in their control. To gain quarian trust Shepard and the true geth must descend onto Rannoch's surface, scare off heretic/husk intruders, and secure a number of reservation sites so that the quarians can begin rebuilding. Legion agrees to return with Shepard to the Normandy to assist him, but if Shepard leaves the system, Legion will return to the dreadnought so they can attempt to maintain an uneasy ceasefire between the Flotilla and the geth armada.
Missions:
Legion is available for all geth-quarian conflict missions, including Rannoch's “Secure: Quarian City”, “Secure: Quarian Farmland”, “Extract: Quarian Marines”, “War and Peace”, and the mission “A House United”.
During the mission “Secure: Quarian City”, Shepard must deactivate a Reaper artifact in one of Rannoch's cities, so that geth ships can fly down and exterminate husk/heretic forces amassing there. Legion is only available as a squad-mate for this mission after the artifact is destroyed. Legion and a company of geth soldiers perform free-fall jumps from geth carriers, into the city to assist Shepard. After their enemies are dealt with, Legion discusses the importance the geth had for maintaining the planet's structures, as a testament of good will towards their creators.
During the mission “Secure: Quarian Farmland”, after the husk/heretics are driven away from agriculture sites, Legion can explain how the geth continued to support the local wild and plant life on Rannoch. They explain that Rannoch has changed very little over the 300 years since the quarian exodus, and that the quarians should have minimal problem adapting to their old environment.
During the mission “Extract: Quarian Marines”, Legion can help Shepard locate the quarian marines fighting Reaper forces in one of Rannoch's abandoned space stations. The sooner Shepard completes this mission, the sooner he gets access to Kal Reeger (provided Reeger is alive).
During the mission “War and Peace”, the geth armada faces off against the quarian fleet over Rannoch. At this point in the story Shepard must support either the geths' perspectives of defense and peace, or the quarians' wish for war and extermination. Legion will act as a voice for the geth consensus.
During the mission “A House United”, Legion brings Shepard to see the birth of the “Geth Sphere”. If David Archer is with Shepard, Legion can use David to upload his organic memories and perspectives to the construct, allowing all geth to finally comprehend an organic's mind. Conversely, Shepard can use David to indoctrinate all geth, including Legion. If Shepard is aligned with Cerberus ideals, he has the option to keep all geth for Cerberus. If Shepard is aligned with the Alliance, he hands over control of the geth to Daro Xen and the quarian fleet.
Appearance:
Legion is largely unchanged in appearance from ME2 to ME3. The only notable difference in their look is that the hole once blasted through Legion's chest has been fully repaired. Legion still wears Shepard's old N7 shoulder piece (which now changes slightly in size and shape, depending on whether Shepard is a man or a woman).
If Shepard indoctrinates Legion with Project Overlord, the geth platform's eye color changes to red.
Notable character interactions:
Legion does not get along well with Ashley or Kaidan. The Virmire Survivor attempts to convince Shepard to either return the geth to quarian control (more of Kaidan's preference), or have the quarians destroy the geth (more of Ashley's preference).
Legion and Tali are good friends, provided they were both loyal in ME2. Tali's newfound confidence in the geth can lead to her and several quarian scientists exploring the Dyson Sphere with Legion as their guide. The geth will also assist the quarians in securing Rannoch from the Reapers, and supplying them with resources.
Legion and Kal Reeger have a poor relationship. Kal is bitter that the geth killed his company on Haelstrom. While Reeger doesn't want all geth to be destroyed, he believes that they should leave quarian space forever, and never bother his people again.
Legion and David Archer (if given to the Alliance) become very good friends. David is seen, by the geth, as a bridge between organics and synthetics. Legion lets David explore the Dyson Sphere, and even plans on building the man a safe and comfortable “home”, once the geth manage to subdue the stormy weather on the megastructure.
Legion is unimpressed with EDI's choices of combat-avatar (LOKI and YMIR mechs). The geth build her a specialized geth platform (mostly identical to the standard geth platform) for more “effective” combat proficiency. Later, through doing side quests, Shepard can aquire special mods so that Legion can provide EDI with a geth-prime avatar.
Potential death:
During Shepard's boss fight with Harbinger on Geth Sphere, Legion and Kal offer to crash a geth/quarian frigate into the Reaper to stop it from uploading a virus to the entire geth network. Legion will actually be able to “hack” its way into controlling the frigate, leaving their avatar with Shepard and company aboard the kodiak. Legion will not be hesitant to die, because in such close proximity to the Geth Sphere their personalities and memories will be archived and dissolved into the megastructure. Shepard must choose between sacrificing Legion or Kal to save the geth.
If Shepard chooses to sacrifice Legion, the geth will give Shepard (or Tali, if she is romancing Shepard) the old N7 shoulder pad strapped to its arm. After the frigate crashes into Harbinger, the geth platform deactivates forever, and falls out of the kodiak to the Geth Sphere thousands of feet below. Legion's body is never found.
Ironing out variables:
If Legion was sold to Cerberus in ME2, then they are found in Cronos Station's docking bay, as a mini boss. Legion kills several of Liara's mercenaries with their sniper rifle, and if STG is involved in raiding Cerberus, they kill Captain Kirahee.
Also notable is that, with Legion in Cerberus's hands, the geth, and Shepard's perception of them, is completely different. The geth are simply “evil machines” of the Reapers. All geth are heretics (since the heretics successfully uploaded their virus to the true-geths' hivemind, forcing them to worship the Reapers) – but of course Shepard and company have no idea about the difference between heretics and true geth. Shepard leads the quarian fleet in launching several nuclear and EMP strikes onto the continent-like structures of the sphere's surface, and the Geth Dyson Sphere is not attacked by Harbinger, but rather defended by the Reaper instead.
If Legion died in ME2 and was (un)loyal, then they are replaced by a geth platform named “Union”. Union is a standard red geth model with a blue “eye” light, carrying a basic geth rifle, and having no personality. They act strictly as the geth ambassador, have no friendly allegiance with Shepard, and use Legion's archived memories to comprehend organics. Union is not rude or kind – they simple are a voice for geth consensus – and can be seen as distant and cold.
[Note: As of right now I will not write a bio for Union. They have no personality traits worth mentioning, are not concerned with sympathy, empathy, or the mistrust that Shepard can show the geth.]
If Legion died in ME2, and was not loyal (because Shepard did not perform their loyalty mission), then all geth are heretics.
If Legion survived ME2 and was not loyal (because of their argument with Tali), then the true geth do not trust the quarians very much, and do not help them secure Rannoch. The quarians replace the geth in all missions where the latter would have aided Shepard on Rannoch, and much of Rannoch suffers horrible pollution from the Flotilla bombing the planet to rid it of husk populations. As a result, quarian scientists estimate that it may be 200 years before their people can readapt to the planet. Tali/Kal do not take off their mask in the quarian subterranean agricultural site, because the air is too polluted. The geth are still loyal to the general war effort.
If both Legion and Kal Reeger are dead by ME3, then Harbinger successfully manages to indocrinate all geth when he connects with the Geth Sphere. The geth become one of the Reaper's massive fighting forces throughout the rest of the story, and can only be released from control when the Crucible is activated. The geth will then promptly begin to attack the old machines.
If all of the geth are heretics, then Harbinger uploads them with Reaper code-- making them more dangerous than ever.