Callista came from the planet Chad, where she lived with her family. They shared the planet with the mouse-like Chadra-Fan. Their business was in moving sea cows along the Algic Current. Although she had been born with strong Force powers, they remained dormant in Callista until one time when she used them to free herself from a floe of pack ice.
After some years, Callista came across another Force user, Geith. Together they travelled to the Moonflower Nebula, drawn there by some unknown power or instinct. There they found one of Palpatine's experimental weapons, the Eye of Palpatine. They realised how evil it was, and tried to destroy it. They died in the process.
Geith died, but Callista used the Force to keep her spirit alive. Instead it stayed in the gunnery computer, and did not allow anyone to use the weapon, thus rendering the weapon useless. The project was abandoned, and so was Callista. For almost thirty years she stayed in the weapon, losing her memories as the years went by.
As this was happening, big changes were taking place in the galaxy. The great Jedi purge had been completed, and the Rebellion was born. More weapons of mass destruction were developed, and some destroyed. Almost a decade after the destruction of the second Death Star, Callista felt stirrings through the Force. She found out that Luke had boarded the Eye, and as a result of an earlier attack was weakened. He saw Callista in his dreams, who felt an instant connection to Luke. She apologised to him for not destroying the station before, a task which Luke meant to undertake then and there.
The Jedi Master had not come alone. Two of his students, Nichos Marr and Cray Mingla, had accompanied him. They, however, could not sense Callista's presence. But although Luke knew Callista was there, he could only communicate with her by using the ship's computer. Through this, Callista showed the three how to destroy the station. It meant that Nichos and Cray would have to sacrifice themselves, and Luke could not bear to lose Callista as well, whom he had fallen in love with.
The plan went through anyway, and as the station exploded, Luke and Threepio escaped in a shuttle. The shuttle met with the Hunter's Luck, a ship owned my Mara Jade. There he met with Leia and Han, and with someone he thought dead. Mara had picked up an escape pod from the Eye. The pod opened, and out stepped Cray, but with Callista's essence inside it. Before the station had been destroyed, Cray and Callista had used the Force to swap body and spirit, so Luke could at last be with the one woman he loved.
During the process, Callista lost her Force abilities, but Luke promised to find a way to get them back. Perhaps the orphaned Jedi Master would not wander the galaxy alone after all.