Rex was a male krogan who was born on the krogan homeworld of Tuchanka. After his father was killed in combat against a rival clan, Rex left his mother and clan, disgracing them, and travelled off world. Having no money or other items, Rex decided to join the oldest profession that the krogan race had known: warfare. Rex, outfitted in his customized armor, became a mercenary, fighting for the highest bidder across the galaxy. His first mission consisted of fighting a gang of human mercenaries who had been hired to protect a turian politician. Confronting them, he killed them all, even though they were nearly all biotics.
Wrex later was hired to fight another group of human biotics users on an uncharted world. Sneaking in to their settlement, he fought and killed them all. He later returned to the person who hired him and he was amply rewarded. On the side he took up bounty hunting, and was hired by another anonymous client. Following the orders of his client, Rex tracked a human to the Afterlife Club on Omega. The trail led through a warehouse on the Citadel, where Rex interrogated and learned the position of his target from a human warehouse worker. After eliminating all witnesses, Rex traveled to Omega and under the alias of Phanx killed his target and returned to his client.
- Mass Effect Origins (First appearance)