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When he tells her what Naga said, the girls make him promise he won't do it. Cynthia, who wants to keep making as many memories as she could with the father she'd barely gotten to know in her own time. Morgan, whose only memories are of her father.
Sumia knows better. When Robin smiles and promises he won't sacrifice himself, she smiles back even as she knows, deep in her heart, that he might not be able to keep it.
Finding out he was Grima's vessel had been a harsh blow Robin has never really recovered from no matter how much time has passed. Every other child has some memory of their parents' deaths, but all Cynthia can remember was that one day her father had simply disappeared. The battles in Plegia, Validar, Lucina's attempt on his life when she realized Robin was the one who'd betrayed and killed Chrom in the future she came from. Seeing the mark of Grima on Cynthia's wrist and looking away in shame, guilt that he'd burdened his own child with such a thing.
She can't magically lift the burden and neither can their children, all she can do is remind him that nothing will ever change how much they love him. That no matter what dark god lurks within him he's still the same wonderful man she'd fallen in love with so long ago.
But her feelings aren't enough to change the facts. The end of Grima means the end of Robin, and if Robin doesn't end it, Grima will forever be a part of him. It's a difficult enough choice without her and the girls constantly fretting.
If you can't keep your promise not to do it, then I promise not to cry when it happens, she says the night before the final battle.
Grima nearly destroys them all before Robin surrenders himself to save them. Everyone calls out for him to return, and Sumia, Morgan, and Cynthia are the loudest. Robin breaks free, runs to their sides, and the battle begins. They can do this, the power of their bonds has proven stronger than any destiny.
Then the moment comes. Grima is weak, his human form on his knees, and Chrom raises the Exalted Falchion to deal the final blow.
Except it's not Falchion that ends Grima's life, but Robin's own sword. Grima howls in pain as he disappears, and barely a moment later, Robin begins to fade as well. Cynthia's eyes widen in horror, Morgan clutches her sister's arm and buries her face in her shoulder.
Sumia runs to him, grasping his hands, feeling their corporeality diminish little by little.
"You promised." He smiles sadly, reaching to stroke her cheek with his translucent fingers.
"So did you, remember?" He brushes his nose against hers, and she tries to smile back even as her eyes burn with unshed tears. A thousand memories of reading together, picking flowers, fighting side by side, flying through the skies on her pegasus, picnics with Morgan and Cynthia, trips to the beach and the hot springs in the Outrealms.
"I love you," she whispers, because that's one thing that has never changed and never will. Love could not guarantee them a perfect ending, but it gave them all the time they had together. She stays with him, holding on as long as she can until he fades into the ether, the last thing to fade his smile.
Later, alone in her room on the ship back to Ylisse, she catches sight of the ring still on her finger, and she can no longer hold back.
I'm sorry, she thinks as she buries her face in her hands, it looks like neither of us could keep our promise.