According to the lore of the Mass Effect series, the geth are a collective of AI (artificial intelligence) beings which do not have the concept of individuality, and think in terms of the entire consensus.

Legion is the first representative from the "true geth" consensus encountered by the player in the role of series protagonist Commander Shepard for the original Mass Effect trilogy.

Legion's backstory involves him retracing the worlds Shepard had visited in the first Mass Effect, searching for the human who managed to defeat Sovereign and the geth heretic faction.

[5] Legion eventually found the SSV Normandy's wreckage on the planet Alchera and learned of Shepard's death, but kept a piece of the Commander's torn off N7 armor and attached it to cover the battle damage in his chest.

[8] The geth were originally created by the quarians, a species of humanoid extraterrestrials known for their skills with technology and synthetic intelligence, to be "servants of the people" centuries before the events of the Mass Effect series.

[12] The design team disclosed that the geth took one of the longest times to finalize, as they wanted to make them stand out from enemies in other science fiction universes.

[2] According to Feketekuty, Legion could connect back to the other clusters of geth consensus at will, though the gestalt intelligence contained with its platform is ultimately an "anonymous collective going out on its own" and is capable of operating under "the equivalent of offline mode".

If reactivated for Shepard to interrogate, he identifies and accepts EDI's analogy of his nature to the biblical story of the exorcism of the Gerasene demoniac in the Gospel of Mark 5:9.

[2] Following the mission's completion, Legion becomes loyal to Shepard and reveals that the heretic faction decided to help Sovereign in the original Mass Effect in return for a Reaper body that all geth could upload themselves into, and join into a single consciousness.

Legion was one of the most voted characters according to the results of a reader's poll published by IGN in December 2014 for their top ultimate RPG party choices.

[26] In early 2018, some media outlets drew attention to the many visual similarities between the reimagining of the 1965 series Lost in Space and the Mass Effect franchise, particular the appearance of the redesigned Robot character.

[27][28][29] In a February 2019 article written for Comicbook.com, Liana Ruppert noted the trending number of Apex Legends players requesting for a Mass Effect Legion skin for the Pathfinder character.

[34] Casey Lynch from IGN believed Legion to have one of gaming's best "first encounters" with a character, which is accentuated by his musical theme and its "rousing crescendo".

[35] Steven Hopper of IGN was excited to see how Legion would develop in the then-upcoming Mass Effect 3 and calling his recruitment "an awesome twist".

[31] GamesRadar's Jordan Baughman cited Legion as an example of BioWare's "Kickass Robot" character archetype, following on from Knights of the Old Republic's HK-47 and Dragon Age's Shale.

He emphasized that Legion's decision to co-opt a piece of Shepard's armor as a crude form of self-repair is an intriguing notion that he is capable of sentimentality.

[6] In his essay "We Are Legion: Artificial Intelligence in BioWare's Mass Effect", Thomas Faller compared the geth narrative of becoming self-conscious about its situation to René Descartes' philosophical proposition cogito, ergo sum and the right to be accepted as individual.

He notes that the Mass Effect universe "does not only provide us with classical themes such as a distinct breach of Asimov's 'Three Laws of Robotics,' but also gives us the opportunity to experience those issues in a more immersive way".

[37] The authors of the 2017 book Ten Things Video Games Can Teach Us: (about life, philosophy and everything), discussed the nature of Legion and the geth, and specifically whether the manner in which their intelligence is manifested fits into American philosopher Hilary Putnam's theory of functionalism.

[2] Dutch theologian Frank Bosman noted that the biblical origins of Legion's name is an example of the numerous explicit references to Christian tradition in Mass Effect.