I got this idea while watching Rogue One, and realizing that Jyn probably had many thoughts in her head when facing her demine that couldn't be expressed through screen alone – her life, abandonment, and her love for Cassian. These are them.
From the look that Cassian had in his eyes, Jyn knew that he was thinking the exact thing she was. They weren't going to make it out alive.
The light from the slits in the walls of the elevator reflected off his brown eyes, making them dance with flecks of gold every half second. She took a moment to appreciate the color as she caught her breath on their way down. They weren't making it out alive, no, but it wasn't for lack of trying. The two had always had a gritty resilience about them, and in the short time they spent together they knew that the other would fight tooth and nail to make it past anything. They were survivors, and even if there was the slimmest chance, they would try to make it. They succeeded in their mission, and they were still riding off that bit of hope they had to continue the fight.
A clunk alerted the two that they had reached the bottom floor, and Jyn took the wounded Cassian under her arm to carry him out. He wasn't even pulling the heroic, "Leave me and save yourself," bit – that's how she knew they were done for. Still, Jyn couldn't just leave him now. In war, you have no choice but to trust the person by your side, and Jyn wasn't giving up on Cassian just yet.
Somehow, as they hurriedly made their way through the base on Scarif, no stormtroopers even questioned them – in fact, it seemed as though they hardly noticed their presence in the heart of their base. Frantic, shuffling footsteps alerted Jyn that they were about to be overtaken by a group of troopers and she heard her pounding heartbeat in her ears, but they just kept on running by them. More groups of troopers converged in front of Jyn and Cassian, and she started noticing they were all heading to the exit. She felt her pace quicken – maybe the rebels had finally gotten the upper hand on the battleground outside. If the shields were down and they were able to transfer the plans, that maybe meant that more rebel ships could get onto the surface, bringing in reinforcements.
Gritting her teeth in determination, she ignored her own pain that pulsed through her body, and shifted Cassian's weight so that she could carry him faster. She felt him move with her, trying to help her as much as he could, and they stepped through the door.
As they limped out onto the beaches of Scarif, they were met with the sight of the full destructive force of the Death Star, and Jyn realized why the stormtroopers were evacuating. Orange and blinding light greeted them, as she felt the heat on her face and knew that they wouldn't be so lucky in escaping the blast like before. Jyn felt Cassian let go, and she followed him down to kneel in the sands, staring at the pure destruction that faced them. It would be there in a matter of seconds, and there was nothing they could do to stop it. She felt her gaze shift up to meet Cassian's, to find that he was staring at her with an intensity, and thoughtfulness, that she had not seen on his face before today.
"Your father would have been proud of you," Cassian said to her in a careful and deliberate way, and she knew he spoke from the heart, not because Cassian knew her father, but because Cassian also felt the same fierce pride in her. She smiled a wry smile at him, always keeping up her cocky persona. Still though, she felt more connected to him in that moment than she had in the entire time she had known him, and found herself curling her fingers tightly around his. He smiled back at her, his eyes warm, fierce, and blazing with a sad light, as he faced death. She wanted to say something comforting back to him, but she realized she had never really gotten to know Cassian Andor, and she never would.
That realization hit her harder than facing her own death. He had risked life and limb to help her on her mission, putting blind trust in her when she could have just been leading him into a trap. Jyn blinked back tears as she realized that nobody, not since her father had been taken away, had put their trust in her like Cassian had. She found herself pulling him close to her, knowing they had nothing else more to lose because their last moments were upon them. She held onto him fiercely, finding herself unable to turn away from the fast-approaching explosion. It was beautiful, in a way, like they were sharing a last sunset together, as the bright light warmed her face and the orange danced across the waves. In her last moments, Jyn was almost fooled, thinking that's exactly what she was looking at.
She felt him, hesitantly at first, pull her closer to him, his strong hands gripping the fabric on the back of her jacket, as he rested his cheek against hers, finding the crook of her neck to shield his face from the blaring light in. Their grip on each other quickly grew strong and confident, as Jyn felt Cassian's surprisingly steady breaths against her. It made her feel steady too; though she knew her fate, it was something she couldn't change. She couldn't change that they would die here. She couldn't change that it would happen in seconds. She couldn't change that she never knew anything about the man that she was holding. She could change how she felt in these last moments, however.
She decided to take comfort in the fact that there was someone here with her. Since she was orphaned as a child, she knew she would be alone. She always was. Inevitably, people left to pursue their own interests, and Jyn was never part of that. This was the fate that she accepted, up until now. Now, there was someone here with her, facing disaster, and they were doing it willingly. Jyn felt overwhelmed with a feeling she never expected to experience again in her life, much less when facing death. Jyn felt undeniable love.
It wasn't love for any reason, or love that could be defined – it didn't need to be. It was not a love that was blossomed out of romance, or the steady heartbeat of someone who was always there. It was not the love of family that pulls you like gravity, and it was not the intense, fleeting love born out of infatuation after meeting someone who was a kindred spirit. It was a love born simply out of the fact that they were there with each other, and they both were willing to die for something, and that love was all they had left. Jyn didn't even consider what could have been between them – there wasn't even the possibility. This, right now, is simply what was.
As the light grew blinding, and the heat grew too intense to bear, Jyn did not find herself looking away. She faced her demise with no regrets, no other thoughts, no other options – she faced her demise with a such a resounding, resilient silence that it drowned out the explosion. She felt Cassian grip her tightly, until they were no more, and together, evaporated into stardust.