Before I begin, let me just say that this entire thing is not linear. I have a lot of affection for this pairing and I'm writing things as they come to me. Sorry in advance. Additionally, there is more to this than I've published and I'm operating under the assumption that Jack has been with the Normandy on and off since around the time Mass Effect begins.
If you're so lovely of a person that you've wandered in here not knowing anything about Torchwood or Jack, you should know that Jack is a man born in the 51st century on another planet. He traveled through time with the Doctor and there was an incident. Since that incident, Jack is unable to die. He experiences injury and 'death,' to a degree, but always returns to himself after a short time. It was easy to insert him into the Mass Effect timeline under the assumption that he simply lived long enough to get there. Also Jack is a really affectionate guy and would basically fuck anything.
Meanwhile, I have crossover headcanon that extragalactic travel was made possible by the reapers after the war, with the assumption that Shepard chose control.
Mass Effect and all associated characters belong to Bioware. Torchwood and all associated characters belong to the BBC. I make no money from this fanfiction.
Jack's expression was blank. Before him, the dusty browns and reds of Rannoch stretched endlessly into the horizon.
A reaper was dead. This was a victorious moment. But as the reaper's voice rumbled across the valley, into Jack's bones, the pieces started to come together.
Sentinels. Nearly two hundred years ago he had been a boy, hearing stories about the sentinels that carried humanity across the stars, away from Earth and through the galaxies. Stories about the sentinels and their shepherd.
Jack felt weak. Shepard was speaking, saying something to the sprawled out and broken machine that used to be a Reaper. But he didn't hear her. The reapers… the sentinels. One in the same. Jack was a product of this. A child of the world after this war.
He flinched at the sound of missile fire. Three quick bursts and the reaper was dead. Like he wished he was. Like he should have been long, long ago.
How was he supposed to face her after this? How could he smile and reassure Shepard and her team, knowing what sacrifices were to come, knowing what devastating losses they were sure to face?
Not long ago he had scoffed at the Illusive Man for thinking he could ever hope to control a power as massive as the reapers. But there, perched before another giant brought down, the man's words sank like a chill into Jack's bones.
"I can control them, Shepard."
He was wrong, of course. The Illusive Man had no chance of control. But Shepard did. And suddenly everything seemed to be falling apart.
Jack clenched his jaw and stared straight ahead as she approached him, hoping to all hell that she didn't see the glistening tears forming in his eyes. She couldn't know.
All too soon he knew he would weep for her.