Jysella was born around 16 ABY. By 25 ABY, she was a student at the Jedi Praxeum on Yavin 4, and she had spent more of her young life being trained there than she had living with her parents.
In 43.5 ABY, Jysella was the third known Jedi to become ill with the mysterious Force psychosis that afflicted both her older brother Valin and Jedi Knight Seff Hellin. During an attempt to seek information on the illness with MasterCilghal, Jysella snapped and immediately saw the Master as an impostor. Unlike her brother, who experienced intense anger, she was motivated by fear. Jysella attempted to breach the temple and when evaluating the situation flow walked into the future and saw her path of escape. This knowledge allowed her to know exactly where the secret security droids where hidden at the temple's entrance, and she quickly disabled them.
Jysella continued to flee the temple, Force hurling pedestrians and screaming in terror that those whom she knew best were not themselves. Two of her best friends, Bazel Warv and Yaqeel Saav'etu, who had also been Valin’s best friends, were on the scene and quickly attempted to stop her once they realized she was afflicted. The two Jedi were unsuccessful and Jysella was apprehended by GA security. The entire scene was recorded by journalistJavis Tyrr. Chief of StaffNatasi Daala arrived soon after and addressed the crowd; explaining that Jysella was now under GA jurisdiction and would be frozen in carbonite alongside her older brother.
When a group of Jedi executed a court-approved document to visit the entombed Horn children, the group, along with the press, discovered the two Jedi displayed over an aquarium in the main office lobby of the Galactic Justice Center. Jysella was eventually freed from her carbonite imprisonment along with her brother Valin shortly before the Jedi Order overthrew the Daala regime, though they both continued to suffer from their ailment, possibly due to their being frozen.
Jysella's age is nowhere closely defined in canon, though the fact that she is not mentioned in Specter of the Past, Vision of the Future or Star Wars: Union suggests that she was born no earlier than 19 ABY. In fan fiction, she is often depicted as being closer in age to her brother Valin than seems possible from canon, which has led to many fans believing her character to be older than she is. However, some fans have also underestimated the range of her possible age, placing her birth closer to 25 ABY than seems implied at her introduction in Dark Tide I: Onslaught.
The preview at the back of the novel Outcast includes an excerpt from the forthcoming novel Omen, which features Jysella as a prominent character. The preview, written from Jysella's point of view, states that Jysella is ten years younger than Valin, making her birth year around 23 ABY. However, when Omen was released, this information was changed to 3 standard years, putting Jysella's birth year around 16 ABY.
On page 362 of the hardcover edition of Vortex, her name is misspelled as "Jysell Horn".