Robb,
First of all, I'm sorry. I was bratty and mean to you. I'm sure that you've had far worse from others since then, but I was your sister. I should have treated you better. Secondly, I'm writing this letter in case I die here. It's very possible, so I'm leaving it with Tyrion. I trust him to send it.
I just wanted to say that I love you. I learned a lot in King's Landing, from people I never expected to teach me. Mostly, I learned that family can get through anything. It's like Father used to say so often, that the lone wolf dies when the pack survives. I think he was right.
I don't blame you for not trying to rescue me, or for refusing to trade Jaime Lannister for me. I'm no warrior, I had no benefit in the war. I do wonder if they told you how I lived here. I doubt it.
You'll make a great king, Robb. Take a lesson from Robert Baratheon and Joffrey, learn from their mistakes. You will be better than either of them.
I always knew you'd be good. You're a good person, Robb, and I worry about you even here. You would have been a wonderful lord, and now you'll be an ever better king, because you care. You care so much about people and justice, just like Father used to.
I suppose I should say goodbye, now. I love you Robb. No matter what happens to me here, that won't change. If you get my body back, please put it in the crypts at Winterfell. I want to be with Father.
Goodbye.
Sansa