Adam Fenix is referred to throughout Gears of War. The games haven't filled in a lot of the details surrounding his work, but it's known that one of his projects included developing a map of the Locusts' tunnel network below the surface of planet Sera.
Adam Fenix is the father of COG soldier Marcus Fenix. Their relationship is an awkward one with small amount of communication and held grudges against the other. For example, Adam Fenix never told Marcus where his mother disappeared to and left him to believe that his mother abandoned them. This is probably not the case.
In Gears of War 2 Adam Fenix is mentioned twice. While in the Locust Queen's chambers, Delta Squad learns that the Queen knows Adam somehow, but no more is mentioned than that. At the end of the credit roll in Gears of War 2 Adam Fenix confirms that he is still alive through a radio message that appears to be directed toward COG command.
In Gears of War 3, Adam Fenix appears in the beginning of the game, informing the surviving Gears that he has a solution to the Locust and Lambent problem, as well as revealing that Imulsion has been responsible for the troubles occuring on Sera. From then on, the primary mission of Marcus Fenix and Delta Squad is to find Adam and help him deploy the tool he has developer to save humanity.
Marcus and the rest of the Delta Squad spend the majority of the game trying to get to Azura, a secret COG base in the middle of the ocean where Adam has been conducting research ever since his "disappearance", which was actually a kidnapping at the command of Chairman Prescott, who tasked Adam to do research on how to defeat the Locust. Once they reach Azura, Adam initiates contact with Marcus and the rest of Delta Squad through the use of security cameras throughout the base. He informs them that he needs the COG's help in administering his solution to the Locust and Lambent problem.
Eventually, Adam reunites with his son, Anya, Cole, and Baird, and after a fierce battle against the Locust Queen, in which Delta Squad protects Adam, he is able to activate his ultimate weapon to eradicate the Locust and Lambent; an energy wave that destroys everything containing large amounts of Imulsion. The weapon works, destroying every Locust and Lambent life-form on the planet, but also ends up killing Adam, who injected himself with Imulsion earlier in his research in order to test his cure. He says goodbye to Marcus and tells him to live a good life, and do his best to help rebuild the human race, before he is killed by the cure.