When they were growing up, Luke and Biggs spent every spare moment together. They were the best of friends. On a world like Tatooine, friendship of this kind was essential. As they grew older, they'd talk about nothing but leaving the dreary desert behind. Luke's uncle made things difficult, though. In time, they made peace with the fact they they wouldn't get to share the fulfillment of their dream. Biggs went away to the Imperial Academy. Luke promised to follow. Who knew what the future would bring?
The Emperor did, as it happens. The Emperor, also known as Palpatine, also known as Darth Sidious, the Sith master. A man who could see the future, quite literally.
The Emperor foresaw the circumstances of his death, and determined that he would do everything in his considerable power to avoid it. He knew Luke would be the cause, and although the boy also served as his last best hope to ensure the future of the Sith, he would be a necessary sacrifice. After all, the Emperor had already lived a very long time, and if he could continue avoiding his death, he would always be around to keep the faith alive himself.
When he heard that Luke's childhood friend had enlisted into Imperial service, the Emperor decided that this man would be the agent of his would-be assassin's demise.
For once, he didn't take his new weapon under his own control. He didn't need to. Biggs Darklighter didn't have access to the Force. Except for his unique ability to betray and murder Luke Skywalker, he was useless.
No, the Emperor had Biggs complete his training as a TIE fighter pilot, but also groomed for eventual promotion to the rank of officer. In that capacity he would have access to high-level planning, and of course assignments, things the lowest levels of the Empire would never even dream of.
One day Biggs was given a deep-cover assignment. He would pretend to defect to the Rebel Alliance, become a pilot of a different craft, what the Rebels called X-Wings. As the Emperor knew would happen, the day came when Luke finally rejoined the side of his friend. They became colleagues, both pilots of the same class of attack ships.
Biggs had become loyal indeed. Afterwards, the Emperor often wondered why it'd been so easy for him to complete the assignment. These were friends, perhaps, who had after all grown up very differently. Privilege always breeds the potential for corruption, the same as poverty. Perhaps that was it. If the roles had been reversed, and it had been Biggs who stood to profit the Emperor in the ways of the Force, Luke would have been similarly suited to carry out the same assignment.
When the Rebel fleet made its assault on the Death Star, Luke and Biggs were among the last surviving pilots.
There was a moment when Luke actually saved Biggs's life. If things had gone differently, Biggs would have died before having the chance to complete his assignment. But things went according to plan. The third remaining pilot, Wedge Antilles, was conveniently taken out of the equation when his ship was damaged. He couldn't stay in formation any longer. That left only Luke and Biggs. Luke was given the task of hitting the target that would destroy the Death Star, which provided the necessary distraction. He never saw it coming.
The Emperor wondered if he'd been right to worry about him after all. Biggs was offered early retirement, which he took. He returned to Tatooine, bought the old Skywalker homestead, and spent the rest of his days amassing a considerable fortune, which he one day spent on an old cargo vessel, the Millennium Falcon, which on the fateful day had arrived a moment too late to affect the outcome. If any any point he'd ever felt regret, Biggs expressed it to no one. He never had a good friend again, anyway...