I quite obviously live at Hogwarts.
Wow. This is, truly, an amazing story. It is written with great empathy for a character we don't usually empathise with. I actually feel a bit sorry for Petunia reading this, and I generally find it difficult to feel sorry for her, given how she mistreats and neglects her own nephew. This story makes her much more of a character rather than a stereotype.
In spite of this, you still managed to capture the essence of Petunia, the little things that make her HER. She is constantly worried what others might think, confused that some don't seem to care about being watched, bewildered by anything abnormal, longing to have security, order and her own little world, slightly jealous still and resentful. From the moment you wrote "I don't joke. Jokes are ridiculous and juvenile.", you had me convinced that this was definitely her. She even corrects Lupin's tenses ("was your best friend") at a time when it is insensitive to do so.
I truly feel sad for Petunia reading this. The new element you added was fear. Reading this, we get a great sense that Petunia was absolutely terrified of anything magical, of the things she couldn't have or fully understand, of the threats that killed her sister. She thinks she isn't grieving, but her fear is strongly reinforced by her sister's death. It confirms what she has known all along. She doesn't want that sort of threat in her life. Even small points of vulnerability such as her childhood nickname disconcert her.
Another thing I found interesting was how you describe her relationship with Vernon. You often wonder why anyone would marry such an aggressive, simple-minded and deeply unpleasant man. It makes perfect sense for your Petunia to seek protection, to go for that sort of "order" and for him "taking charge", providing her with a sense of belonging and being "taken care of". Dudley means the world to her, and she will spoil him.
We can even see a bit of courage in Petunia's act of going to her sister's funeral, putting on that black dress and getting the car. How sad that she wasn't able to listen, to be open, to be a part of the magic through Lily. This is really heartbreaking to read and very persuasively written.
Your Lupin was also in character, young and terrified, alone and shaken up by recent events, embarassed about his appearance but partly beyond the point of caring. Dumbledore is strong, trying to console everyone, finding the right words. Imagining interactions between these three has a bizarre quality, but it works here.
Author's Response: Wow, thank you so much! This means a lot, I was a bit wary in writing Petunia, especially in first person which I don't normally like to do, but I'm very glad you enjoyed it. Thanks for reading!
One more note: "I walked towards my car, shaking off the grief that people had left on me."
That was my favourite sentence in the entire story. It works so well because it shows that even the grief is somehow "dirty" to her, a burden, something that clings to her, something indecent. She doesn't feel comfortable with it and she would never allow herself to grieve for her sister. It is other people's grief she needs to remove like stains from a white blouse.