David Archer

David Archer is a brother of the Cerberus scientist Gavin Archer and an exceptionally talented mathematician.

Mass Effect 2: Overlord
Project Overlord was a Cerberus research project based on Aite intended to devise a way to control the synthetic race called the geth. It was discovered that David was able to communicate with the geth by reproducing their patterns of speech. His brother Gavin Archer, chief scientist of the Project Overlord, concluded that it was David's autistic mind that made this possible and made him a part of the project.

Throughout the story of Project Overlord, David was subjected to dehumanization and exploitation under the sentiment of doing the universe a favor. To Gavin, learning to control the geth was a goal that justified everything he was putting his brother through, which eventually led him to force David to connect to geth network turning him into a hybrid of a human and a Virtual Intelligence. Over time 'Overlord' VI turned rogue and killed all the scientists involved in the project except Gavin.

Commander Shepard was sent to investigate the research base. While still stuck in the machine connecting him to the geth network David managed to show Shepard his memories and lead them to where he was located. In order to escape David attempted uploading himself into Shepard's ship. Shepard stops the process and deactivates the program.

If Shepard stops the experiments, David is subsequently removed from the care of Cerberus and his brother and taken to Grissom Academy, an Alliance educational institution for gifted children.

If the Commander is willing to sacrifice David for science, David is given medical treatment, stays in his brother's care, and Project Overlord continues.

Mass Effect 3
If Shepard saves David in Mass Effect 2 they can meet him again in Grissom Academy during the Cerberus attack on the school. If Garrus/EDI are in Shepard's squad they will comment how David looks much healthier. He vouches for the Commander's trustworthiness to his fellow students, and also gives the Commander access to a room containing guns in it. After escaping Grissom Academy, David goes with Kahlee Sanders to assist in building the Crucible.

If David was left in the care of Cerberus, his brother explains that they were able to calm David down and were able to get some useful data on controlling the geth, but eventually David stopped responding to stimuli. Dr. Archer euthanised his brother as an act of mercy.

If Shepard never dealt with Project Overlord, then Gavin Archer informs Shepard that David died when Cerberus used nuclear weapons to destroy Atlas Station.

Autism
David is easily overwhelmed by loud noises.

He tends to comfort himself by counting numbers, solving math problems out loud and repeating the same sentence. He also stims by rocking his body back and forth as well as hand rubbing.

He has an excellent long-term photographic memory and he's able to deconstruct other languages from observation. He loves math and everything revolving numbers.