- "She was aided by Carth Onasi, a decorated war hero of the Republic and a legendary soldier. During the Mandalorian Wars he was honored many times for his bravery."
- — Saul Karath to Darth Malak
- "I'm not a warrior, I'm a soldier. There's a difference. Warriors attack and conquer, they prey on the weak. Soldiers defend and protect the innocent – usually from warriors."
- — Carth to Canderous Ordo
Shortly after the Mandalorian Wars, the former Republic hero Revan returned with a Sith armada to battle the Republic. Amongst the Sith forces were legions of treacherous Republic forces, and one name stood out from the rest; Admiral Saul Karath.
To Carth, Saul was a mentor and hero, and so Carth was devastated when, in a test of his Sith allegiance, Malak, Revan's apprentice, ordered Saul to bomb Carth's home world of Telos, due to its strategic significance to the Republic. Carth lost his wife, and his son disappeared. From then on, Carth refused to trust anybody, lest he suffer the same sting of betrayal again.
Carth eventually found himself aboard the Endar Spire along with Bastila Shan. The ship fell under attack above the Outer Rim world of Taris, and Carth, a single Republic soldier, and Bastila escaped to the planet below. Carth and the soldier co-operated in an attempt to find and rescue Bastila, which they eventually achieved, thanks in no small part to the soldier's growing connection to the Force.
Upon their escape from the Sith bombardment of Taris, Carth became pilot of the newly acquired Ebon Hawk, which the group then used to reach the Jedi Enclave on Dantooine. As Bastila discussed the matter of her Force-sensitive companion with the council, Carth expressed his disgruntlement with being 'left out of the loop'. Eventually it was decided to train the Republic soldier, who Carth then accompanied on the search for the Star Forge.
Eventually the ever-growing group was seized by Leviathan, Saul Karath's vessel. Carth soon ended up face to face with the Admiral, yet a force-cage barred his path to revenge. The resourceful team soon escaped imprisonment and confronted the Admiral on the bridge. Carth was finally able to exact his revenge. Saul, in his final moments, revealed a devastating truth to Carth; that the Republic soldier he had befriended was none other than an amnesiac Revan, former Dark Lord of the Sith. Carth's hatred of the Sith welled up, but was soon eased as the party discussed the new revelation, and Revan rejected his former personality as the Dark Lord.
During the quest, Carth also encountered his lost son, Dustil Onasi, on the Sith planet of Korriban. Carth was again confronted with his hatred of the Sith when it was revealed that his son had joined them, blaming his father for failing to protect them. The party was finally able to convince Dustil of the evils of the Sith, and Carth was relieved when his son abandoned the dark side.
Carth went on to participate in the Battle of the Star Forge, eventually becoming an Admiral of the Republic fleet in the aftermath of the Jedi Civil War. As Revan departed for the Unknown Regions, he imparted to Carth the task of keeping the Republic strong.
Carth was forced to later live up to not only his task, but also his past, as the Sith attacked Telos for a second time, this time under the command of Darth Nihilus. Carth swore that he would not see Telos devastated again, as he led the Republic fleet into combat. Soon after the battle, Carth requested an audience with the hero of the battle, the Jedi Exile. He questioned the Exile about the reasons for her exile. Wishing the Exile a safe journey, he asked her to look for Revan in the Unknown Regions and said that, should she find him, tell Revan that Carth is awaiting and following orders.Behind the scenes
In Knights of the Old Republic, if the player decides to reassume the title of Dark Lord of the Sith via the non-canonical ending of the game, Carth flees away in terror when he is told of the change on Rakata Prime. Consequently, he does not appear in the sequel if the player says that Revan followed the dark side, being replaced by Admiral Cede, because if the player chooses the Dark Side option Revan leaves the planet on the Ebon Hawk, which would be Carths only possible means of leaving the planet.
Just as the male Revan can engage in a romantic sub-plot with Bastila, the female Revan can with Carth. And if the player sets Revan as a light sided female in Knights of the Old Republic II, then Carth will express his feelings for Revan to the Exile. Also, Darth Traya will comment that it would have helped had she made him understand, but a hero of the Republic, no matter how brave, cannot understand war as Revan did.
If the player has both romanced Carth and turned to the dark side, then the player will have the option of killing Carth herself or have Bastila finish it. In a cut-scene that didn't make it to the final game, Revan had a third option to kill Bastila and die aboard the Star Forge with Carth. It was not placed in the final version because the developers did not have enough time for a finishing game.
In Knights of the Old Republic II, if the Exile has enough influence with HK-47, then the droid will make a verbal mockery of Carth, based on Carth's distrust of people in the original game:
- "Mockery: Oh Master, I do not trust you! I cannot trust you, or anyone else ever again!"
Carth was voiced by Raphael Sbarge.
Carth also had his own blaster pistol. It was in close design to the standard Republic Blaster. It's also fully upgradeable.