EDI

EDI (pronounced 'Eee-Dee' [ˈiː.diː]), which stands for Enhanced Defense Intelligence, is the AI of the Normandy SR-2 and is represented visually by a holographic floating blue sphere and aurally with a feminine voice at various terminals throughout the ship where she can provide basic information about the Normandy and Cerberus.

EDI is voiced by Tricia Helfer.

Mass Effect 2
EDI is a Quantum Blue Box type AI that functions as the electronic warfare defense for the Normandy SR-2. Because of the potential danger of a rogue AI, she has been given behavioral blocks and cannot interface with the ship's systems. She also serves as the Illusive Man's eyes and ears on the Normandy, monitoring the many listening devices on board and sending regular reports to him.

The Illusive Man restricted Commander Shepard's access to some of EDI's files and capabilities for unknown reasons, although EDI surmises that the reason is probably because he does not want Shepard to know everything about Cerberus just yet. It is only later in the game, after acquisition of the Reaper IFF and its integration with the SR-2's systems, that a virus disables the ship and enables the Collectors to board and capture the entire crew. While Shepard and the squad are away, Joker grants EDI access to the ship's full array of systems, allowing them to escape. EDI will also reveal that the Normandy SR-2 was built using technical diagrams of the SSV Normandy, which was built by the Systems Alliance upon the encouragement of Cerberus. The SR-2 was assembled using parts bought from various vendors, and built by Cerberus in a remote area of the Voyager Cluster.

EDI also gains access to "Anti-Reaper Algorithms" later in the game, and states that she devotes significant processing power to analyzing them. When pressed on this subject by Shepard as to how she could hope to combat beings millions of years more advanced, she reveals that she was in part designed by technology gained from Sovereign's remains and thus, at least partially, based on Reaper technology herself.

At the end of the game, Shepard has the choice to defy the Illusive Man and leave Cerberus. If this is the case, the issue of the numerous tracking and monitoring devices Cerberus has on the Normandy is not elaborated on, although it appears that Shepard has earned EDI's loyalty and, as an AI (now having full access to the ship), she would seem to be free to stop sending reports to the Illusive Man.

Personality
As an AI, EDI's behavior goes far beyond that of typical VI interactions, with the ability to act independently and interact on a social level with the crew- not unlike any other sentient being.

Her most important relationship is with Joker. As the Normandy's pilot, he is directly responsible for the ship's well-being, and, by extension, hers. At first, Joker is apprehensive of EDI, believing that starships can operate well enough without an AI's assistance. However, as the game progresses their relationship improves, though they pick on each other. After the attack by the Collector Cruiser on the Normandy, EDI states that she and Joker have developed an "equitable working relationship". After the final mission, EDI states that they have developed a kind of platonic relationship, which she surmises is not related to any kind of hormonal attraction on Joker's part. This is reflected in how they address each other- at first EDI calls Joker "Mr. Moreau", but later starts calling him Jeff, and Joker at times responds to EDI as "mom", reflecting his viewpoint of the relationship. After the Collector attack, Joker also begins referring to EDI as "her" and not "it".

EDI appears to have an understanding of humor, as she has made numerous comments of an intentionally humorous nature, especially after being unshackled during the Collector intrusion into the Normandy. She appends such comments, or humorous comments made by others, with the phrase "That was a joke." This sense of humor could also be described as poor, as her jokes—delivered in a deadpan manner—are often disconcerting rather than funny. After EDI interfaces with the Normandy, she is taken aback by her new found sense of freedom to experience the world around her, saying that the change is like having worn gloves all your life only to suddenly have them removed. Philosophically, she likens the ship to being her body, the drive core her heart, and the sensors her eyes.

Mass Effect 3
EDI will return in Mass Effect 3 and will be present on the Normandy SR-2. At some point, EDI will gain control of an "infiltration unit" that serves as a robotic body.

Trivia

 * During certain points on missions and while talking to her on the Normandy, she will have some unique, if not humorous dialogue.
 * EDI may be the most advanced AI in the Mass Effect Universe (Maybe apart from Ilos)