Urdnot Wrex

Born into Clan Urdnot, Wrex quickly gained fame for his prowess in battle. He became a leader of one of the smaller Urdnot tribes while still a youth – the youngest krogan to be granted the honor in a thousand years. After the Krogan Rebellions ended, most remaining krogan tribes wanted to continue fighting despite the genophage and its devastating effect on their ability to reproduce. The most influential voice calling for a return to arms was one of the few warlords to survive the war, Wrex's father Jarrod. Wrex had different ideas. He believed that the krogan didn't have the numbers to go to war, and even if they did, the genophage ensured they couldn't replenish their numbers quickly enough. Instead of battle, he led his tribe to focus on breeding, for at least one generation, and called for other tribes to follow. Gradually other krogan tribes began to come around to Wrex's ideas. This angered the Warlord Jarrod, who decided to arrange a Crush (a meeting on neutral ground) with all of the tribes. Wrex, sensing it was a trap, reluctantly agreed. He explains this to Shepard by saying "when your father invites you to a Crush, well, there are some laws even we hold sacred."

Wrex and his best warriors met with Jarrod's group in the krogan ancestral burial grounds called the Hollows, a holy place among the krogan where violence is forbidden. When it became clear to Jarrod that Wrex wouldn't agree with him, he signaled his men. They leapt from their hiding places and ambushed Wrex's warriors, cutting them down. Wrex managed to escape the ambush, but not before sinking his knife deep into Jarrod's chest. With his father's betrayal Wrex's eyes were opened to the truth about the krogan: most would rather die in battle than try to rebuild their society. Realizing the proud warrior culture that once valued courage, strength, and honor – values embodied by the Krogan Monument – had now been reduced to glorifying pointless violence, Wrex turned his back on the rest of the krogan and left.

Over the past three centuries, he has served no master but himself; working as a bodyguard, mercenary, soldier of fortune, and bounty hunter. Wrex has had some interesting adventures — including an encounter with a fellow merc named Aleena – and he was even hired briefly by Saren Arterius to raid a volus cargo ship, though Wrex didn't realise who the turian was until he met Shepard. Tales of his exploits have even made it as far as the quarian Flotilla. Even after many travels, there is little in the galaxy that can still surprise him. Despite his brutish appearance, Wrex rarely loses his temper. The mere threat of his anger is usually enough to get what he wants. When his fury is unleashed, however, it is a truly terrifying sight. He doesn't speak often, but when he does his words are direct and often shockingly blunt — that tends to get people to listen.

(borrowed from the Mass Effect wiki)