I, for one, believe in the headcanon that Leia actually had a giant crush on Cassian. (She’s 16 and Cassian is 23, she’s not really romantically intrested in him but he is attractive, and a good man and loyal rebel. So it happens.) But Leia, being Leia, did an excellent job of hiding it.
But Cassian, being Cassian, being the best spy in the Rebellion, picks up on it.
And Leia knows he knows. (And he knows that she knows that he knows. And so on.) Which makes it harder for her, but still. (Maybe it was this practice that gave her the strength to look Vader in the eye and not betray the Rebellion. Maybe.)
They keep it between themselves. It’s never spoken of. Cassian understands, really he does (it’s normal, he shrugs it off), he finds it endearing, honestly. And he’s quite surprised at how well Leia is able to hide her too-long glances, the way she dampens her reaction when Bail offhandedly mentions his name. He has respect for the Princess. She makes direct, unwavering eye contact with him on the odd ocassion he needs to speak with her, or her father.
It wears off, eventually.
Leia’s 19, is handed the Death Star plans. The Death Star is blown up. They commemorate the fallen. She’s always made a point of remembering each and every single being that has contributed in any way to every mission. The Rebels don’t mind the long list of names (one day, they might be a name on that list.) that their Princess recites.
(Galen Erso, Bodhi Rook, Jyn Erso…)
She sees Cassian’s name. Her heart skips a beat. She doesn’t pause visibly, nobody notices. (Except maybe Luke - her twin and Jedi-in-training, after all. But maybe he’s too excited about the award ceremony to notice.)
She says his name, recognizes his sacrifce, remembers it (along with the others), carries it forever. And she moves on.