Of them all, Edmund understands Susan the best.
He knows what it's like to be given a choice between Aslan and the White Witch, between Narnia and Other. He understands what it is to want something completely different from Narnia, and he remembers how it feels to be betrayed by the choices you make. Edmund chose Aslan last, the White Witch being his first choice, although in the end he was nothing more than a pawn to her. He'd loved her despite everything, not because she gave him sweets and promised him power, but because she listened to him when no one else would. Susan chose Aslan first, and she hasn't quite forgiven him for her rejection yet (Edmund doesn't think it was a rejection, not really, but it's not his opinion that matters).
And it's her right, to change her mind, to choose a different path.
Because that's the thing about choices: just because you make one now doesn't mean you can't make a different one later.
Of them all, Lucy forgives Susan the easiest. She doesn't quite understand how Susan can choose to forget about Narnia and being a Queen (for Lucy, there has only ever been Narnia, only ever been Aslan, even before she entered the wardrobe), but she does understand that there are two sides to every story. It's why she's always failed multiple choice tests. There's never just one answer, with no gray area in between. Susan's options aren't limited to either A. Choose Narnia or B. Forget Narnia. It's more complicated than that. This answer requires reams of paper and a mind strong enough to write down the heart's thoughts.
It's not that Susan has chosen to forget Narnia; it's that she's chosen to believe in something else more than she believes in Narnia. Susan's choice might not be the right decision, but that doesn't mean it's not a right decision. Lucy believes in Aslan, believes in him absolutely, but she believed in her siblings first; and just because she believes in Aslan now doesn't mean she's forgotten who taught her what faith was. Lucy knows that adventures lie in wait for Susan, adventures she herself will never experience because the only choice she's only ever been able to see is Aslan; she never had an B, C, or D, much less an essay portion.
One day, when Susan meets up with them again, Lucy will listen to her story and hear about all the possibilities you miss out on when you only see one of the choices.
Out of them all, Peter knows Susan best. They've always been the elder siblings, the ones who took care of the children when the war stole their parents away. So he knows that when Susan mocks him for still believing in Narnia, when she critiques Lucy for having little interest in appearance, when she berates Edmund for his social awkwardness, it's only because she's still trying to watch over them. But they have grown apart and it's hard now not to take offense at every comment.
Susan glitters, diamond bright and as seemingly unbreakable, but even diamonds have flaws and she always looks so tired. Susan would have been (is) a great mother, and like all mothers, she worries about her children. The problem is, for a while, her children encompassed not only Edmund, Lucy, and even, at times, Peter, but all the people in Narnia, and when they went back for their second and final time, she'd found out that not only had her children grown up, they'd grown up thinking they'd been abandoned; left, without warning or explanation. Susan has always blamed herself, even though she could have done nothing to prevent it.
Peter thinks they were led out of Narnia because children are supposed to outgrow their parents, and Narnia's people were ready to live on their own again. They'd relearned pride, freedom, and independence. Kings and Queens hadn't been needed any longer, and there comes a time when all parents have to let go. Aslan had simply expedited the process.
The thing is, parents never fully forget their children. So Susan will continue to fuss at him, Edmund, and Lucy, and someday she will come back to Narnia and let herself be forgiven by the children who were left behind.
Out of all them, Susan has always been the most ruthless. No one can rule a kingdom without getting their hands dirty, and Susan was willing to do whatever it took to keep Narnia safe. Oh sure, Edmund would arrange political assassinations from the shadows, Lucy would laugh even as she trailed the blood of their enemies into the castle, and Peter would sentence criminals to death with barely a moment's regret, but Susan is the one who watched, who told Edmund who needed to die, who shot arrows into the condemned. People often took the title of Gentle at its surface value, and never looked at what laid beyond it.
So when Susan stands in the morgue among the bodies of her dead siblings, she doesn't cry. She looks at them, at their peaceful faces, and she wants to curse, wants to scream, but instead she turns sharply on her heel and leaves the room. Outside, she breathes in deeply, huge heaving gasps that shake her body like the tears she won't allow herself to shed.
'Is this what you wanted?' she thinks. 'Is death how Aslan rewards his followers?' They were so young, regardless of the fact that they'd lived two lifetimes. Aslan had reverted them, made them into children again, but just because he'd given them another life didn't mean he had the right to take it away. Peter, Edmund, Lucy, she thinks; Magnificent, Just, Valiant.
What a tragedy that no one ever got to be Wisdom. Perhaps Aslan was afraid of the competition. After all, if one of them had been Wisdom, then Aslan's all knowing, sanctimonious speeches might well have been challenged. Or maybe, Susan conjectures bitterly, Aslan knew Wisdom is not something that blindly accepts things on faith.
But Susan has long since stopped letting a mere title define her. She can be Gentle, but she can also be wise, kind, intelligent, funny, bitter, joyful, frivolous, and anything else she wants. She's no longer limited to one title, one defining characteristic. Now she'll be Just, Valiant, and Magnificent as well, because the people who never stopped claiming those titles were taken from this world for the good of another. Isn't that always the way it goes?
So maybe she's embraced the title of Queen Susan the Bitter, at least for the moment, but she's more than earned that right. Her parents, her siblings, everyone she loves is dead, and for what? An ideal world that has plagued them since they were children?
Nothing is perfect. She of all people knows this, knows that memories change and distort with time; even so, even knowing this, part of her will always long for Narnia. Nevertheless, Susan doesn't regret not getting on that train; she has a life here, a good life, and one she's happy with. She'll live it out, live it to the fullest extent, for herself and her siblings; and when she's done, she'll damn well make sure she gets into any afterlife she wants.
After all, once a Queen of Narnia, always a Queen of Narnia.