It begins with a game.
They are children of resistance fighters, but they are still fundamentally children. There are dozens of them scattered through the city in little pockets of ones and twos, and they gather on the solstices and equinoxes in quiet parties to remind themselves and each other of who they are, and that there is hope.
No one at the parties uses their real names, in case someone betrays them, but they mostly know each other anyway, and no one would do that anyway. This year they gladly kiss when the others point out that they are flagrantly violating tradition bynot kissing. Bren laughs into her mouth and Tilda smiles and they pretend not to know each other's names. They pretend the kiss wasn't anything at all.
But Tilda writes her letters, on small scraps of paper she can glean, tidily stitched together to form something of a rough sheet, detailing every individual thing she would like to see and do with Bren at her side, and by the next party, they don't even try to pretend their kisses mean nothing at all. They still kiss everyone else, though. Neither of them would like to be rude.