This is a letter from Ryuko to Soji, found at the back of the Siren 2 Maniacs book and known as the "101st archive". It appears in an actual envelope that must be opened to read the letter inside.
When I was with you, I was happy.
I had always believed that I wasn't normal.
But when the 'real' me awoke, I was so scared I couldn't bear it,
and I wanted to disappear.
Because like I always said, I was really a monster.
If you hadn't given me your leather jacket when we met for the first time,
my body would have melted away and vanished in the morning sunlight.
No matter what, you always accepted me.
I know what a kind person you are.
I wanted to be with you so much I rejected my mother's call.
I wanted to have you by my side, smiling that childish smile of yours, forever.
By the time you read this letter, I might be gone.
However it happens, don't be sad if I'm not there anymore.
Your smile was like my sun.