Leffbruk was born to his father and mother and spent his earlier years travelling throughout the galaxy. After losing his mother to a shuttle accident when he was a baby, he continued exploring with his father on their ship, the Silverstreak. During the Eiram and E'ronoh War, Dass and his father had to deal with many threats. They were caught in a blockade, detained by security forces, and chased by pirates who were using the conflict to prey on defenseless ships.
Around 382 BBY, the Leffbruks joined fellow prospector Radicaz "Sunshine" Dobbs in a search for a paradise planet.
Eventually Dobbs stranded them on the planet Gloam. Spence was injured attempting to pry loose a piece of hull plating from some wreckage and had to stay at their camp while Dass worked on his own to hunt for food while avoiding the monsters which prowled the planet. He would set out in a direction to get a lay of the land and catch scrabblers so they would have something to eat. He explored the ruins of an old city. Getting nervous, he returned to their campsite. Spence was glad to see his son return, and apologized for getting them stranded on the planet. Dass dismissed his father's apology and took a look at the makeshift emergency beacon he was trying to assemble. As it got dark, the two moved inside the cave they had setup as a shelter. Dass continued to explore and hunt and returned to the camp after a fruitless journey. Tired, he paused outside the cave only to realize the creatures had changed their strategy and come early, using an incoming storm to hide their approach. He and his father fled into their cave and blocked the entrance with a piece of metal plating. A monster punched through the barricade and grabbed Dass but was driven off when Spence jabbed it with an improvised weapon. Dass realized that their distress beacon was unprotected, outside the cave.
After staying up all night, Dass snuck outside to retrieve the beacon, but was ambushed by a monster. He jabbed it with the prod, but was unable to drive it off. The young man was saved when his father threw rock at the monster, injuring it. Once safely back in their cave, Dass revealed that the beacon was undamaged.
Shortly afterwards, Dass was present when their distress signal was followed by Mittik and Dietrix Jago who found their cave. Initially skeptical, he and his father warmed up to the strangers who provided them food and medical care while the Leffbruks explained their situation. Dass listened while his father described their trip to a paradise planet and subsequently being double-crossed by Sunshine Dobbs. Dass told Mittik and Jago about how wonderful it felt to be on the planet. When Dass told the group that he had been to the ancient city, he volunteered to lead the group there so Jago could continue to search for the missing Pathfinder team.
The group arrived at the ancient city and Dass became nervous about entering it. Jago suggested the Leffbruks could wait outside, but the boy did not want to risk being stranded on Gloam again and decided the best plan was to stick together. Dass assisted Jago with hotwiring an old pneumatic crane so they could reach one of the entrances. He was amused by the pilot's enthusiasm. Once the crane was operating again, he entered the basket and was lifted up to an archway leading into the city. Looking down, he became nauseous due to the height and held on the railing of the basket. Overcoming his fear, Dass jumped onto the ledge and joined the group to explore the city. As they made their way deeper into the city, Dass and the others discovered tapestries that told the history of the Katikoot people. The party descended deeper into the city and were confronted by a large swarm of monsters coming up from a central pit. Dass took Spence's prod and used it to drive off several of the creatures that were trying to attack them. When a monster grabbed the prod and tried to drag him over the side of the pit, Dass released it and sent the monster tumbling down. He and the others quickly made their way into a small alcove as they tried to escape from the swarm.
Trapped in the alcove, Dass embraced his father and thanked him for taking him on his exploration of the Galaxy. He confided with him that he wanted to join a Pathfinder team if they survived. Before the monsters could reach him, the Jedi arrived and attacked them. Rooper Nitani introduced herself to the Leffbruks and told them to be ready to run once the Jedi had cleared the path to the outside of the city. When the path was cleared, Dass ran as fast as he could with the other towards the Pathfinder's ship that was parked outside the city. Reaching the Umberfall, Dass and the others were horrified to see Rillik begin to transform due to the miner's curse and quickly boarded the ship. The boy was able to escape Gloam with the Pathfinders and Jedi and get to safety on Aubadas.
Several days later, Dass and Nitani sat by a waterfall in Diurna and talked about their future plans. The Leffbruks would accompany the Pathfinder team to Batuu where Dass and his father would try to find a way to recover their ship. He joined the celebration that erupted when a ceasefire in the Eiram and E'ronoh War was announced.
On Batuu, Dass's friend Sky Graf of the Graf family approached him and Nitani to ask them to help them find the mysterious paradise planet, called Planet X. Dass hoped to recover recordings of his mother from the Silverstreak there, and Graf hoped their father was still alive after crashing there. They planned to locate the planet using a special compass Graf had that could direct you to a place that you wanted to go if you had something from that location, Graf hoping that Dass himself, having been to the planet, and some help with the Force by Nitani could get the compass to work. They also joined an event called the Hyperspace Chase as a cover to hide their intentions from both Graf's family and Spence. However, they eventually decided to abandon their mission in favor of helping save lives during the Night of Sorrow, a battle between the Jedi and the Path of the Open Hand, by restoring communications over Dalna, the planet where the battle was taking place. Dass and Graf also both moved on from their reasons for wanting to get to the planet, and both decided not to continue trying to find it.
- Star Wars: The High Republic Character Encyclopedia