My dearest Elphaba,
I was your roommate for a good long time, as you well know. We spent many nights sharing our deepest secrets and desires, our hopes and fears, and I can't recall how many times I dragged you along with me as I snuck out of our dorm late at night to spy on Fiyero. I do, however, recall that one particular night where we hid crouching in those bushes in the middle of a horrendous rainstorm. Though I admit we were not outside for very long, my memory of your condition when we returned to our room is quite clear. You were positively soaked to the bone. Therefore, in my opinion, it isn't plausible that water could have melted you, wicked or not.
I know you have an awfully good explanation for lying to me and allowing me to believe my best friend to be dead. It wasn't until things had settled down from your "demise," the Wizard's departure and my new position as ruler over all of Oz that I allowed myself to sit down and mull things over in detail. I realized then that you must have known it would be best to disappear, or better yet, "die." We both knew that the people of Oz would not accept you back into this land peacefully, not after the Wizard spent so long destroying your reputation. I am truly and deeply sorry that I did not stand up for you sooner, but in the end we're both alive, the Wizard is gone and you're free to live your life outside of Oz. As I promised you, I have done nothing to try to clear your name, however much I wish I could.
Boq came to see me the other day. It seems he has been having the most terrible feelings of regret over his ruthless actions that he believes aided in your death. He explained to me what happened with dear Nessarose (bless her soul) and I figured you must have turned him to tin to allow him to survive without a heart. I wish I could find a way to change him back, but as we both know, spells cannot be reversed. But there is some good news yet involving magic! My attempts at decoding the many mysteries of the Grimmerie are going well. You remember the Green Elixir? It seems it is not from our world, and so (though this may not have been the greatest foresight I have ever displayed) I took the tiniest of drinks and found a few rogue spells become clear before my eyes. Luckily for me, one of such spells was a spell for clear vision, which, with a little personal modification, allows me to better understand the strange language of this fascinating book.
This brings me to my next point, the book and its origins. As you know, the Wizard brought it over with him from his homeland, but it was not all that he brought. Maybe you caught my hint earlier? The Green Elixir. I recognized it the day you left with Fiyero, though I didn't fully understand it until I confronted the Wizard the day you left. He had a bottle as well, and that leads me to a single conclusion... the Wizard is your father. Your mother was having an affair with the man who would later order your execution. I know this cannot be easy for you to read my beloved friend, but I hope now you understand the true meaning behind your powers. You are a child of both worlds. It gives you the ability to perform magic with no formal study, to read the spells within the pages of the Grimmerie, and it is the Green Elixir that your mother drank excessively that caused your... unusual skin colour. If nothing else, let this truth serve as clarity to the questions surrounding your life.
I suppose that unless you should return some day, most of this information will be of little use to you. I just want you to know that you are not forgotten, and I feel no malice towards you for any deeds you have committed in the past, whether with good intentions or wickedness in mind. I meant what I said in the castle, meant it with all my heart. It is because of you that I am who I am today. You may not have been my only friend, but you were the only one that mattered. But then again, you were always more than a friend to me. You were my sister- you ARE my sister, and you always will be. It may be that we will never meet again, not in this lifetime, and if that is to be so then all I ask is for you to know this. I will remember you and your brave sacrifices for the rest of my life. I will remember the times we shared, the times that seemed much too short but I am eternally grateful for. Ozspeed, dear sister...
Wishing you all the best, Glinda