Basically, Dumbledore didn't come and get Harry during the summer after his fifth year. This story is Dudley writing to Harry about ten years after they parted at Number 4 Privet Drive in DH. Just something that popped into my head.
Dear Harry,
I don't know if this letter will even reach you. I know we haven't spoken in nearly ten years, but I've been thinking about you recently.
Do you remember that night in the kitchen one summer? It was a few days before your sixteenth birthday, I think. Something bad had happened that year when you were away at school, but you never did tell me. You had been different that summer – quieter and there was a deeper anger simmering just beneath the surface. I didn't think much about it. Until that night.
I had been hungry (like always) and I snuck down into the kitchen at about two to grab some biscuits. You were sitting down in the kitchen and you were talking to yourself. Your words were too quiet for me to hear the conversation you were having with yourself, but I heard you muttering to yourself "Just do it. That's it. Just do it." I heard something about you making everyone happier. You turned around and opened a drawer and I stepped into the kitchen.
When you turned around, you were holding a long knife and you looked at me in shock. I don't think I had ever truly looked at you since you had returned from school. You were pale and haggard, your eyes dull and framed by circles so dark they were almost black.
You said my name then and before I even had time to think, I had picked up the telephone and dialed the police. It was an instinct that they had ingrained in us since primary school – to call for help if things were becoming dangerous – and it was all that I knew how to do. I didn't even particularly care for you, but still I picked up that phone.
The police showed up at the front door a half hour later, their gruff voices demanding, "What's the issue?" and I didn't even quite know how to answer. I just let them inside to where you were still frozen in the kitchen. Mum and Dad were furious with you, of course, but you were spared from their anger for a few weeks at least while they took you away to the mental hospital.
I know you were angry with me. I know that that night probably made you hate me even more, but that's okay. I want you to know that I never regretted that night. I still don't know why my first instinct was to pick up the phone, but I'm glad that I did. I don't know what you were going through, but it wasn't worth going through with your plan.
I was – and am – your older cousin. I should have been like an older brother to you when we were kids, and I wasn't, and I'm sorry. I'm sorry for bullying you. I'm sorry for not standing up for you at school and even at home. You didn't deserve everything that my parents did to you. So I'm sorry.
I know I haven't seen you in almost ten years. I'm married now, and I have a son who just turned two – William. I know you've got a couple of kids yourself now. I wouldn't be opposed to getting them together for an afternoon if you don't mind them mixing with their crazy, non-magic relatives!
Regardless, in light of this news about your family, I wanted to let you know that I don't regret protecting you, and I will always be there to protect your kids, regardless of whether or not you want me near them and regardless of whether or not they need me. But if anyone hurts them, they'll have a 130kg man at their defense. Because family sticks together. Family protects each other, and I regret that I never did that for you.
I hope this letter finds you happy and well. Say hello to your wife and kids for me!
Your cousin,
Dudley Dursley
Let me know what you thought! :)