Sidonie Garen was a Jedi Master of the Jedi Order, identified as a Human and female. During the Cold War era, while stationed on Alderaan, Sidonie succumbed to a severe illness induced by the Sith Lord known as Vivicar, which resulted in her descent into insanity. This occurred at a particularly inopportune moment, as Alderaan was playing host to a clandestine peace conference involving all the noble families, and Sidonie actively tried to incite further conflict. The Jedi Council dispatched the Jedi Consular, who would later become the Barsen'thor, to locate her; this Jedi was successful in curing her, thereby liberating the Jedi Master from her mental instability.
During the Great Galactic War, Sidonie embarked on a mission to Malachor III alongside Eriz Vossan, Yuon Par, Cin Tykan, Duras Fain, and Parkanas Tark. However, while on this planet, Garen and her fellow Jedi discovered a dreadful evil and narrowly avoided being influenced by the spirit of Terrak Morrhage, a deceased Sith Lord from the Old Sith Empire. As they formulated a plan to escape the dark planet, Vossan lagged behind; Parkanas rescued him but was consequently trapped and abandoned by the other Jedi. Garen carried the weight of responsibility for Tark's loss.
In 3643 BBY, Master Garen was stationed on Alderaan. This planet, formerly part of the Republic, had descended into a civil war following the deaths of Queen Silara Panteer and Prince Gaul Panteer. A corrupt noble, Bouris Ulgo, a descendant of Trask Ulgo, had then seized the throne. She had dedicated herself to brokering a peace agreement among Alderaan's noble houses, hoping to end the conflict. However, before completing her task, she fell victim to Lord Vivicar's plague, raising concerns within the Jedi Council when she ceased communication.
As the peace summit approached, Alderaan received a visit from Yuon's Padawan, a gifted Jedi Consular, who was searching for all the Jedi Masters affected by the plague. When contacted via holo, Sidonie quickly dismissed the Consular as a mere Padawan and refused to disclose her location. Somehow anticipating that the Consular would seek out House Teral, the only house without a delegate for the summit, and that Teral would endorse the Consular by forming an alliance with House Organa, Sidonie hired House Rist to abduct Lew Organa before he could meet with the Terals. However, Lord Teral informed the Consular, who went to rescue Lew. Afterwards, they discovered Sidonie's holofrequency on the Rist base's computers and get the Jedi Master on the holo. Exposed, Sidonie made a hasty exit.
As the summit commenced in the Sendoth Ruins, Sidonie instructed the guards to prevent anyone else from entering. During the summit, the plague-ridden Jedi Master had already started to bring war on Alderaan due to her madness. Overwhelmed by the plague, which made her aggressive, she became convinced that war was the only path forward for Alderaan. She presented incriminating evidence about each of the Houses at the summit, reminding them of old grievances and fueling the escalation of the conflict. When the Consular managed to pacify the delegates, the enraged Garen confronted the Jedi, unleashing her Force abilities. The Consular defeated her in combat and was able to use the shielding technique to save the insane Master. Regaining her senses, Sidonie noted that the Consular seemed weak from shielding her. Apologizing for her actions, Sidonie left handling the summit to the Consular and reported back to the Jedi Council.