Mass Effect: Foundation
When Kolyat was born, his father proclaimed him a gift from Arashu. However, Kolyat rarely felt Thane's presence throughout his childhood, with his father too clueless at parenting to even acknowledge him.
Kolyat knew of the love between his parents, watching them from a door crack when Irikah supported Thane's decision to freelance his assassin skills. This unfortunately resulted in Kolyat being raised by his mother as Thane's jobs kept getting longer and farther.
One day, his father's enemies made a move on them. Kolyat hid in a safe place and was instructed by his mother to stay quiet until she returned. Kolyat was never found by the intruder but he never saw his mother alive again afterward. While in custody of crime scene investigators he witnessed Thane's grief and subsequent breakdown upon the latter's return from a mission.
Mass Effect 2volus bankers on the Citadel as an inheritance for Kolyat. However, Kolyat acquired it early and took up a job as a hitman. He was hired by Elias Kelham from the information he got from Mouse. He was to assassinate a turian politician named Joram Talid in the 800 Blocks.
Kolyat stalks Talid, gaining his window of opportunity just outside the turian's home. His initial attack misses by a small margin as the turian's Blood Pack bodyguard gets shot instead. Commander Shepard and Thane have been monitoring Talid's movements from separate locations in order to stop Kolyat from becoming a killer.
If Shepard and Thane fail to timely update each other, Kolyat succeeds in killing Talid and escapes in a X3M. Otherwise, he is interrupted by the Commander, who shouts at him by name. Kolyat chases down a fleeing Joram and forces him into the execution position, just as Thane and Shepard arrive. They stop Kolyat from assassinating Joram either by giving Kolyat both a warning shot and a punch to the face, by shooting Kolyat in the arm, or by shooting Joram.
Kolyat is furious at the interruption and at seeing Thane again, bitterly aggrieved at his father's absence from his life even after his mother died. He finally calms down as Thane explains why she was killed and why he couldn't bear coming back, beginning to shed tears when his father calls him the only good thing he ever added to the world, having taken many bad things out of it.
Thane and Kolyat are given time to speak in the interrogation room in the C-Sec Office. Kolyat can do community service, work for Captain Bailey, or go to trial. In any case, Thane stays in contact with Kolyat to make up for his mistakes.
Mass Effect 3
Kolyat later appears by his father's side after Thane is mortally wounded by Kai Leng during a Cerberus attack on the Citadel. Despite giving his father a blood transfusion, the final stages of Kepral's Syndrome make it impossible for Thane to recover.
Kolyat is present when Shepard arrives to visit Thane on his deathbed. As his father is too weak to finish a prayer to Kalahira, the drell goddess of oceans and the afterlife, Kolyat continues for him, leading his father to remark that he "speaks as the priests do". Kolyat then confirms that he has been spending time over the last year with the drell priests, seemingly per his father's wishes.
Kolyat then mentions that he's brought a prayer book with him, and asks Shepard if they would like to pray with him. Regardless of Shepard's response, Kolyat leads a prayer for redemption. On its completion, his father passes away.
Shepard states some confusion over the prayer. A male Shepard questions why Thane would still seek redemption, as he died a hero; a female Shepard questions why the last lines referred to a "she". Either way, Kolyat reveals that his father had already made peace with himself, and that, as was his dying wish, the prayer was intended for Shepard instead.
Mass Effect 3: Citadel
Kolyat emails Shepard requesting a memorial service for his late father. If the Commander accedes to the request, the service is held in their Personal Apartment, with Thane's old Normandy crewmates and the salarian Councilor attending. During his turn to speak, he reveals how he thought his father was going soft, but after the way recent events turned out, he thinks maybe his father had everything figured out after all.
Before leaving, Kolyat informs Shepard that he has been going through Thane's bequeathed possessions and has found three video messages that he has sent to Shepard's extranet address, but were intercepted while the Commander was incarcerated.