A Human male, Karlsu joined the Republic Military sometime before the Cold War between the Galactic Republic and the Sith Empire. By the year 3643 BBY, Karlsu had achieved the rank of Lieutenant and was stationed on the planet Taris. He served under Captain Childress at Waypoint Station Aurek, as part of the Taris Resettlement Initiative's security forces.
When Republic forces identified the remains of the Endar Spire, the Republic warship that Darth Malak had attacked over 300 years ago in an attempt to kill the Jedi Knight Bastila Shan, Captain Childress saw an opportunity to help the Republic. Childress believed that the logs of the Spire, which had witnessed Malak's bombardment of the planet, would prove invaluable in defending against an attack like the one on Taris or the Sith's more recent Sacking of Coruscant. He sent a team of researchers, accompanied by a commando unit, into the ruins to retrieve the ship's logs.
However, the wreckage had been already found by a group of pirates and scavengers, who attacked and killed the commandos. The researchers survived, however, and Childress sent another team into the Spire to drive out the pirates. The captain had sent a total of five teams into the ship by the time a Republic-allied individual arrived at Waypoint Station Aurek, and Karlsu was a member of the fifth and last group.
The individual, who had solved a land claim dispute in the Resettlement Project's main city of Olaris, was asked by Private Sakal to contact Childress. Childress asked the individual to find the researchers and help them retrieve the logs, so they travelled to the wreckage and entered. The individual found Karlsu near the entrance, and the lieutenant, upon realizing that they were an ally, signalled the remaining researchers to come out from hiding.
Under the leadership of the individual, Karlsu and the team made their way through the ruined ship to the main computers. Karlsu and the individual fought off the pirates while the researchers retrieved the logs. After they defeated the pirates, Karlsu thanked the individual and told them that he would get the researchers and the data back to Aurek safely.
Lieutenant Karlsu was created for Star Wars: The Old Republic, a massively multiplayer online role-playing game released by BioWare on December 20, 2011. He appears in the Taris mission "Wake of the Spire", where Republic players escort him and a group of researchers through the pirate-filled ruins of the Endar Spire.