Around 2102, Kolos started serving as an advocate and thirty years later he served on Narendra III. Kolos grew old watching Klingon society change, and the pursuit of honor not in valor, but in bloodshed, and the Klingon legal 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")
Kolos was played by Star Trek veteran J.G. Hertzler. It was the first Klingon character he played since Martok and his first named character since Laas, both of whom he played on Star Trek: Deep Space Nine.
The script of "Judgment" described this character by stating, "Kolos is older, probably in his sixties, with a grey hair and beard. We can sense immediately that he's not typical of the Klingons we're used to; he seems more dignified and thoughtful. But there's also a weariness about him. He's a veteran of the Klingon judicial system, and we'll discover that over the past decades he's witnessed the gradual degeneration of Klingon society into a brutal warrior culture. This pains him greatly... but the fire and fight have gone out of him." Also, in an ultimately unused line of dialogue from the script, Kolos commented to the other Klingons during Archer's trial, "If I had a word of advice to give Captain Archer, it would be to keep out of our affairs..." 
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.