From: Children of the Jedi (Barbara Hambly), Darksaber (Kevin J. Anderson), and Planet of Twilight (Barbara Hambly)
Callista was a brave Jedi Knight who lived years before Luke Skywalker was born.
Raised on the water planet of Chad III, she grew up on a ranch run by her father and stepmother, until the call of the Force led her to be discovered by the Jedi Master, Dijinn Altis.
At a young age, she left Chad and followed Dijinn to Bespin, where she, along with other students, learned to be a Jedi.
30 years before she met Luke, Callie and another Force-sensitive, Geith, infiltrated an Imperial dreadnaught known as the Eye of Palpatine. The Eye was used to destroy families of the Jedi, and Callie and Geith were determined to destroy it. However, both were killed in the attempt. Somehow, Callie managed to have her spirit infuse itself into the Eye's gunnery computer, where she kept the dreadnaught dormant for 30 years, until Luke came aboard.
Her life would never be the same when Luke stepped on board the Eye. Sensing her presence through the Force, Luke was able to communicate with her, and they saw each other in their dreams. Eventually, the two Jedi fell deeply in love with each other.
Callie got a second chance at life, when Cray Mingla, Luke's student, decided to sacrifice herself to destroy the Eye once and for all, and offered her body to Callie so that Callie could finally be with her beloved Luke.
However, Callie paid a terrible price: she lost her ability to communicate with the Force. Distraught, she felt she was incomplete and unworthy of Luke's love. Despite their best effots, she and Luke were unable to get her to touch the Force.
Worried that she would be tempted to turn to the dark side to get her powers back, and that she would only be a source of pain in Luke's life, Callie made the ultimate sacrifice and walked away from the love of her life, promising to return to him when she was whole once again.