Dustil Onasi was the Force-sensitive son of Morgana Onasi and war veteran Carth Onasi. He was taken by the Sith during the bombing of Telos IV, and later trained at the Sith Academy on Korriban before turning away from the Sith and returning home.
Dustil was born on Telos IV and raised mostly by his mother due to his father's constant absence as a soldier in the Republic Navy. During the bombardment and destruction of Telos IV's surface during the Jedi Civil War, Dustil's mother was killed and he was assumed dead. In reality, he had survived (and according to him, was captured), but developed a strong dislike for both the Galactic Republic and his father for their inability to save Telos IV.
Some time later, a friend of Dustil's named Selene convinced him to join the Sith Academy on Korriban. There he embraced his hate for the Republic and his father and quickly became one of the more promising students there. He was strong with the Force and proficient in combat, and soon attracted the watchful eye of the academy's headmaster, Uthar Wynn.
While training, Dustil developed a relationship with Selene, who was also a student at the academy. However, Selene was not as promising as Dustil. This distressed Master Uthar, who felt that such an emotional entanglement would hinder his best student's progress. The conniving Sith Master secretly had the young woman murdered and then lied to Dustil about the occurrence, saying she had died on a mission in the valley. All of these experiences just pushed him more towards the Sith, as he felt the Sith cared for him and that they were his family. This also made him think that his father never cared for him, and that his father was the sole reason his mother was dead. As a result he had severed his family ties to his father. At some point he met Jordo, an old friend of Carth, who was making a stop on Korriban while working for Czerka Corporation. Dustil didn't recognize Jordo, but Jordo did recognize Dustil.
During the Jedi Civil War, Carth learned from Jordo that Dustil was still alive on Korriban. He and Revan, a former Sith Lord, infiltrated the Sith Academy to try to find the Star Map on Korriban, while Carth also accompanied to find Dustil. Carth soon found Dustil but was hurt by the choices Dustil had made. He tried to talk with him and convince him to turn away from the Sith by telling them that they were evil. Dustil refused, still wanting to blame his father for his mother's death, and believed the Sith to now be his true, caring family.
However, Dustil chose to allow Carth a chance to prove that the Sith were evil while giving his word that he would not inform anyone that he knew Revan and Carth were at the Academy under false pretense, provided they did nothing to endanger his standing with the Sith. When Revan and Carth found a journal of Master Uthar's in his private chambers detailing the events of Selene's death, they decided to show it to the young man as proof that the Sith were truly evil. With this newfound knowledge, an emotionally distraught Dustil was convinced to forsake the Sith and their teachings.
Dustil promised to eventually meet his father on Telos IV, where they could begin to repair their relationship. Instead of leaving the Academy immediately, he decided to remain on Korriban to try and help as many of his friends as possible and to find some information inside the Academy that could help the Republic. Carth wished him luck, though had doubts as to whether his son would ever be the same.
According to his father, Dustil always hated being deceived. Carth also recognized that Dustil is stubborn and never did anything by half measures, something father and son had in common.
His father and the Republic's inability to save Telos and his mother were the core sources of his anger and hatred. When Carth, traveling with Revan, found Dustil at the academy, he nearly attacked his father for not being present when Telos IV was bombed by the Sith.
Although he was following the ways of the Sith, he was not yet ready to completely sever his connections to his past though he was relatively close. He still kept his surname, though preferred to use only his first name; use of his last name angered him. When he met Jordo, he didn't recognize him. When Carth found Dustil, Dustil immediately expressed his disdain for Carth. On the other hand, Dustil still remembered his mother and thought well enough of her to claim her as family.
Dustil was still able to make some friends in the Academy and even developed a romantic relationship with Selene. After realizing that the Sith killed Selene, he was shocked that the Sith had been lying to him all the time, for he had been a loyal student. Because of this, it was likely that he would not allow the Sith to deceive him again.
Dustil was strong in the Force and showed remarkable progress in being able to use it and was skilled in the use of his lightsaber. This had impressed Uthar Wynn. It was known that he was able to use Force drain and Force Horror and basic Telekinesis as some of his abilities in combat.
Dustil (as well as the whole secondary quest) appears only if Carth has told the player about his family and afterward the player visits a spaceport with Carth where they are informed about him.
While the above is the canonical light side version, it is possible for the player to kill Dustil if the player is following the dark side path, or simply chooses the wrong dialogue options while Carth is present. Much like Juhani in the Dantooine Grove, aggressive comments will provoke Dustil into attacking you.
In Star Wars: Knights of the Old Republic II: The Sith Lords, Dustil was supposed to make an appearance in the tomb on Korriban. Dustil would claim that he is a Jedi and turned his back on the dark side only to return to Korriban for reasons unknown even to himself. While inside the tomb, Dustil would claim to have encountered and killed Carth twenty two times while other times, Carth had killed him. While speaking with the Jedi Exile (who he believes is just another vision) he would cry for his father and ask why he left him. Dustil would then become enraged and attack the Exile. However, the voice for the scene was never done, and was cut from the final version of the game. Instead there is a body labeled "Dead Jedi" in the chamber with a datapad belonging to a Jedi named Nebelish. The designers never changed the model for the corpse however, and he still clearly resembles Dustil.
- Star Wars Knights of the Old Republic: Prima's Official Strategy Guide
- Star Wars: Knights of the Old Republic Handbook
- Knights of the Old Republic Campaign Guide