How Did Garrus Get His Scar in Mass Effect?

Garrus Vakarian got his scar on Omega in Mass Effect 2, when a Blue Suns gunship caught him at the end of Shepard's mission to rescue the vigilante known as Archangel and a rocket hit him in the face. He nearly died. The Normandy's doctors patched him up with cybernetics, and the damage to the right side of his face stays with him for the rest of the trilogy. (Mass Effect 2)

Why Garrus was on Omega#

In the first game Garrus was a frustrated officer in C-Sec, the Citadel's police, and joined Shepard to bring down Saren. After Shepard died and the galaxy went back to ignoring the Reapers, he gave up on the rules altogether. He went to Omega, the lawless station run by Aria T'Loak, built a squad of twelve, and started killing the gangs that ran it. Under the name Archangel he was so effective that three rival mercenary groups, the Blue Suns, the Blood Pack and Eclipse, put their feud aside to finish him off. (Mass Effect; Mass Effect 2)

One of his own squad, Sidonis, sold the rest out. The gangs struck while Garrus was away and killed everyone else, leaving him alone and pinned down in his hideout. (Mass Effect 2)

The fight where it happens#

Shepard arrives on Omega with a dossier that names Archangel as a recruit, without knowing who it is. The mercenaries are hiring freelancers for the assault, so Shepard signs up as one and walks straight through their lines. Inside, Archangel takes off his helmet and turns out to be Garrus. (Mass Effect 2)

What follows is a long siege. Shepard and Garrus hold the building against Eclipse, then the Blood Pack, then the Blue Suns. Before the fight, Shepard can sabotage the attackers in several ways, including damaging the Blue Suns gunship while its mechanic is working on it. (Mass Effect 2)

The scar comes in the final wave. Tarak, the Blue Suns leader, brings the gunship back. In a cutscene it swings round to the window where Garrus is firing, hits him with its guns and then with a rocket, and he goes down. The player can't prevent it. Shepard has to finish the gunship off, then finds Garrus lying in his own blood, barely breathing and still holding his rifle. (Mass Effect 2)

How he survived#

Shepard's squad gets him back to the Normandy SR-2 for treatment by Dr Chakwas. When he next turns up, in the Normandy's comm room, about a quarter of his face on the right side is badly damaged and covered by a dressing, and Jacob Taylor mentions that cybernetics were needed to keep him alive. A chunk of his armour's collar has been blown off as well. Garrus is his usual dry self about it. (Mass Effect 2)

He then moves into the Normandy's main battery, where he spends most of the game calibrating the guns. The scar stays on his face for the rest of Mass Effect 2. (Mass Effect 2)

The scar after Mass Effect 2#

By Mass Effect 3 the wound has healed into a permanent scar, and the game makes a running joke of it. Garrus jokes that the krogan-turian alliance could be sealed by marrying him off to a krogan female, since they like scars. James Vega calls him "Scars" while the two compare war stories in the mess hall. (Mass Effect 3)

It matters in the romance too. A female Shepard who romanced Garrus in Mass Effect 2 touches the scar in their private scene, and in Mass Effect 3 Garrus jokes that she always did like scarred men and offers to get more if it helps his chances. Shepard can carry on the relationship by kissing the scar. See Mass Effect romance options for how that path works. (Mass Effect 2; Mass Effect 3)

There's a reasonable reading, and it is only a reading, that the scar is the visible cost of the Archangel years. It marks the point where Garrus stopped being a by-the-book cop and became the soldier who would follow Shepard anywhere. The games never say it outright, but they never let it fade either. (Mass Effect 2; Mass Effect 3)

Read more in the full Garrus Vakarian biography and the Mass Effect 2 story explained. (Mass Effect 2)