Kolos was a veteran Klingon advocate in the mid-22nd century. His father was a teacher and his mother a biologist; his parents advised him to study law. Fifty years before 2152 he started serving as an advocate and thirty years later he served on Narendra III. Kolos grew in his old years watching the Klingon society change, and pursuit of honor not in valour, but in bloodshed, and the Klingon law system to become gradually a tool for the Warrior caste.
In 2152 he defended Captain Jonathan Archer on Narendra III when the captain stood trial after being accused by Duras of hiding fugitives aboard the starship Enterprise NX-01 and conspiring against the Klingon Empire.
Years of witnessing dishonorable conduct had left Kolos somewhat cynical and he initially provided Archer with a weak defense. He tried to broker a deal with the magistrate, who was willing to offer Archer a deal: If the captain gave up the location of the fugitives, his life would be spared. Archer challenged his ethics and expressed outrage that Kolos wasn't fighting to attain justice. Kolos explained that he became an advocate many years ago, when the courts honored justice and fair play. Now they ignored these things in favor of the warrior code. He believed that he was too old to fight for change. Archer accused him of not being an honorable Klingon. This inspired Kolos to offer a much more aggressive defense.
In the courtroom, Kolos challenged the state's witnesses and obtained permission for Archer to answer his accusers. He turned the court's attention to how Archer had assisted the Empire in the past, such as exposing the Suliban plot to overthrow the empire and rescuing a Klingon ship caught in a gas giant. The magistrate still found Archer guilty, but agreed to spare Archer's life on these grounds. The magistrate commuted the death sentence to life imprisonment on Rura Penthe. When Kolos protested that this was still a de facto death sentence, he was sentenced to a year there himself.
When Malcolm Reed arrived to rescue Archer, Kolos declined an offer to join him, saying he hoped to inspire changes in the Klingon courts, which he would be unable to do as a fugitive. (ENT: "Judgment")
The Pocket ENT novelization of The Expanse makes it clear that Archer had no contact with Kolos between the time they separated and Enterprise's entrance into the Delphic Expanse, with Archer unsure of whether Kolos was still alive.
In the novel Beneath the Raptor's Wing, it was established that Kolos had served out his sentence and had returned to Qo'noS by 2156. He had to relay to Captain Archer the news that the Council had decided against providing assistance to Earth in their war against the Romulan Star Empire. Kolos did tell the captain that, while there would be no official support for Earth, the Klingons might be able to provide more subtle forms of assistance.