Mass Effect: HomeworldsEdit
When Garrus arrived on Omega, he found Sidonis in one of the lower levels of Afterlife, being beaten by an angry krogan affiliated with the Blood Pack. Garrus stepped in to help, and the two of them fought off the remaining mercs. Afterward, the two of them decided to form a team to fight off the gangs. After assembling a team of 10 other vigilantes, they significantly lower Omega's crime rate.
Sidonis was eventually captured by the Blue Suns and coerced into luring Garrus to another district by giving him a false lead on the Blood Pack. When Garrus leaves to investigate, the Blue Suns attack his hideout and kill the rest of his squad. By the time Garrus discovered his dead teammates, Sidonis had already vanished.
Mass Effect 2 Edit
Garrus eventually learns of Sidonis' general whereabouts and that his disappearance was facilitated by someone called "Fade". Garrus then asks Commander Shepard to help him exact his revenge. Together, they discover that "Fade" is, in fact, the former C-Sec officer Harkin.
After interrogating him, they force Harkin to arrange a meeting between Shepard and Sidonis. Garrus sets up a sniping position to assassinate Sidonis. Shepard has the choice to facilitate the assassination by leading Sidonis into Garrus' line of sight or warn Sidonis of his assassination attempt.
If Shepard helps Garrus, then Garrus will shoot Sidonis with a sniper rifle, killing him.
If Shepard chooses to warn Sidonis, he is revealed to be suffering from survivor's guilt as a result of his unwilling betrayal, eventually apologizing to Garrus via Shepard's communicator, and mentioning that he'll make it up to him somehow. It's later revealed in a news report on the Citadel that Sidonis turned himself in to C-Sec, confessing to the murders of Garrus' team. However, it is uncertain if he will be convicted, as the murders were committed on Omega and outside the Council's jurisdiction.