I am sorry that I have made you believe that I hate you. Trust me, little brother, I don't I'm only harsh on you because one day, I won't be there to protect you, and I want you to be able to fight without me. We all know I'm going to die before 40. It's true.
When we were kids, I could tell you always looked up to me. You always said how "strong and invincible" I was, and how you wanted to be like me one day. Yet you didn't understand. The only reason I was strong was for you. You were always so innocent, I made it my job to make sure no one would hurt you, but now it seems that I'm the one who hurts you. And I'm sorry for all those times.
When we were nine, I got thrown into depression. After several years of that hell, I wanted to die... at the age of twelve. I would be dead right now if it weren't for one person: you, Mikey. You're the only reason I'm still alive right now. Sure, you piss me off by being annoying, but I still love you more than anything. I want you to be stronger than I ever was to and for you.
Mikey, you're like the sun. No one can live without you. Once you're gone, you're gone. Everything dies. And I'll die first. I can't live without my sun. You're my sun, Mikey. My sunshine. I might sound cheesy, but I'm telling the truth. When you come back, you light everything up with that stupidly adorable smile of yours, and you chase away the darkness. Maybe that's why you choose to hang out with me a lot. You probably know of my darkness, my demons. But you make it all better. You're life to me. You're the only reason I'm alive: you gave me hope for something better when I thought all that I could be good for is to go and die. Maybe one day, someone will figure out a way to live without sun, but it will never be the same. When you're gone, nothing and no one can ever replace, Mikey. You're unique. You're special. You are a wonder. You are my sunshine, Hamato Michelangelo.
He snuck into his little brother's room that night and carefully laid the letter in Mikey's hand. Hopefully, it's the first thing he sees. He needs to know the truth. He doesn't expect one back; he's not that great of a brother.