RebelCaptain

Rebelcaptain is the het ship between Jyn Erso and Cassian Andor from the Star Wars fandom.

Rogue One: A Star Wars Story
Jyn Erso, daughter of Galen Erso, meets the rebel spy Cassian Andor when the Rebellion breaks her out of jail in the hopes that she can provide an introduction to Saw Gerrara, her adoptive father, and figure out the location of Galen.

Cassian and Jyn are suspicious of each other at first -- her being suspicious of anyone, and him being suspicious that she could be a liability on their mission -- but as they begin to work together, it quickly becomes clear that they have a natural chemistry. In their short time knowing each other, they pull out each other's contradictions and convictions (Cassian getting Jyn to realize she does care about the Rebellion, Jyn getting Cassian to realize he needs to follow his gut and not his orders) and both show each other a better way of being.

Cassian goes back for her several times when it isn't strictly necessary for the mission, prioritizing her wellbeing at a higher level than he is seen doing for others. When she discovers he was originally slated to kill her father, they argue bitterly, with her accusing him of being basically a stormtrooper and him calling out how convenient it is for her to decide she cares about this cause now.

But when Jyn fails to convince the council to back her plan, he shows up with a group full of soldiers, pledging to follow her into battle, at which point she thanks him for sticking around, which she isn't used to.

During the Battle of Scarif, they fight side by side. After he appears to have been killed by a blaster shot, she continues on alone, only for Cassian to return at the last moment to save her from Orson Krennic. She sends the Death Star plans which they have stolen up to the fleet above them, and they hold each other up as they limp to the lift.

In the lift, they share a moment of rest, leaning together and gazing at each other's faces.

When they stumble out onto the beach, they can see wave of destruction from the Death Star already heading their way, and they know they aren't going to make it out alive. Instead, they collapse together on the beach to watch it approach. She reaches for his hand. They embrace, and are still holding each other tightly when the light of the explosion swallows them.

Star Wars: Andor
A prequel TV show to Rogue One starring Cassian Andor came out in 2022. Although he does not meet Jyn Erso until the events of Rogue One, the show has featured cameos from Saw Gerrara, and many fans suspect we will hear about or see Jyn during season 2, although the characters are unlikely to cross paths themselves.

Fanon
This ship came into being pretty much as soon as audiences started leaving the theatres after seeing Rogue One. Those who ship it believe it is a clear case of two characters who would have fallen in love if they only had the time to do so, and see their ending as tragic and romantic. Others see this relationship as purely platonic, with some expressing relief that the movie didn't force the two into an explicitly romantic confirmation that wouldn't've have rang true with the condensed timeline they have to get to know each other.

Fandom
Cassian/Jyn works on AO3

Quotes
"Trust goes both ways."

- Jyn to Cassian

"Your father would have been proud of you, Jyn."

- Cassian to Jyn

Trivia

 * Actor Diego Luna has frequently expressed support for the two as a romantic pairing.
 * The Rogue One novelization contains several passages of internal monologue that show how inexplicably drawn to Jyn Cassian is: "Was it concern that made him fixate on her? […] She dominated his thinking nonetheless. Cassian believed neither pity nor pragmatism explained it. […] Maybe it was the need  he’d seen in Jyn, the fire that had carried her through the fighting in the Holy Quarter. It seemed obscene to leave that need unanswered, abandoned to the dust."