Hey Saeli! Finally here with your review! We've already talked about how sorry I am for taking Forever, hehehe, so I'll get right into for you!
In your request you wrote that you wanted me to give you an 'honest to God review', so that's exactly what I'm gonna do!:
I think your story is grabbing right from the top with the letter as your opening. It throws us immediately into the plot and you supply us with these details and insights to later chapters that perk up our interest straight away. You mention things like Muggles knowing about the war and Hermione in St. Mungos which makes me go 'Ooo!'. Also, I found it very clever of you to throw your OC at us straight away - no descriptions about who she is, no big explanation chapter that I see in a lot of OC stories, just right to it. We simply accept that she is friends with the trio and Lupin and Kingsley because it's thrown onto us, no questions asked, and it really works for you here!
"I wish I could tell you that everybody is fine, but only some of us are." - I really enjoyed this line! It has a sadness to it, but it's not dramatized or stretched out, it's simply telling the truth about their situations. Love.
Now I know that you know this already, but your grammar is really holding you back. I only picked out examples that no one else who reviewed this chapter have mentioned yet, alright?:
"but I greatly disagree to (with, not 'to') that."
"but I don't know until when (how long, not 'until when') they can keep at it."
"The healers are not sure on (of, not 'on') what curse he used"
"but it won't be long before you could (can, not 'could') come here again"
"the news about Hermione hasn't (hadn't, not 'hasn't') sunk in yet, even if (though, not 'if') he received the letter almost a fortnight ago."
(The next 2 are just sentences where you repeated a word too close together in a way the distracted and took away from the story)
"he heard a knock on the door. He quietly placed the utensils in the sink and crept to the door" - Repetitive with 'door'
"...Moonlight sonata played in the piano. Lupin lead him to the study where he found Willow at the piano" - Repetitive with 'piano'
(I would simply consider finding an alternative word for both of those so you aren't quite as repetitive)
"Harry looked around his room, its (it had, not 'its') been almost two years since he (not 'he', either he'd or he had) left this place. Although he hated it to be just here (Although he hated being here) , but (he, not 'but') somehow in his heart he (no he) still has (had, not has) a sentimental tie to this place. Harry heaved a sigh and stood from his bed. He wore a shirt over (threw a shirt on over, not 'wore a shirt over') his bare chest and went down to the kitchens. He opened the fridge and ate the last piece of cake Hagrid (had) sent him for his seventeenth birthday." - So aside from all the technical errors I pointed out, this paragraph really doesn't do it for me. I just 100% cannot see Harry ever feeling sentimental or attached to that house. I would honestly suggest that if it's not vital to the later plot, you consider returning to canon for this part, where Harry has no attachment to the home. I mean, he was treated terribly here! He grew up in a broom closet under a set of stairs, and when he finally did get his own room, they began locking him in it some days, feeding him 3 small, cold meals a day and only letting him out to use the bathroom! He never even got so much as a sincere Happy Birthday from them, so why would he ever feel attached? It's just not believable by any stretch of the imagination UNLESS you are not sticking to canon where Harry's history is concerned. What I mean is that the only way this could work is if your Harry in the fic did not grow up abused and treated like dirt by the Dursley's. But if you're sticking to canon with his relationship to them, then this just doesn't work. Also, the cake thing - Harry would never be allowed to keep cake in the fridge, and not just because Dudley would eat it all the first chance he got, but because the Dursley's would never allow that. Remember, Harry used the loose floor board in his room to hide food. Or are the Dursley's no longer living there? If that's the case, you need to say so in the chapter!
So I have to ask: Why is Harry offering Lupin a cup of tea? Again, Harry hates that house and would not want to linger either. And he's never offered any visitor to the Dursley's house anything before, so I doubt he'd start now, you know?
As far as your OC and my first impressions: Well, I don't know much about her yet, but I'm gonna say this: A lot of FemOC's that we read about in fanfiction are the 'tough, don't follow orders out of loyalty to a friend' type characters (which Willow seems she might be, based on her disobeying orders to write Harry) and they have special talents (like the ability to play a difficult song, like Beethoven, on the piano). These are two of the only traits I even know about your OC at the moment, but they're immediately sending signals that point right to 'Mary Sue'. Now, the truth is I could be very wrong. She could be nothing of a Mary Sue after reading later chapters, but it's important for you to understand that the first impression we are getting is that she's going to turn into a Mary Sue, and that is probably turning a lot of readers off right from the top. Even if she's truly not a Mary Sue by next chapter, she is in this one, based off of only two facts. Does that make sense?
Alright, well I really believe that if you tidy all of that up, this could be really great! I'm intrigued and want to know what happens next, but I can see a lot of readers turning away because of the things I pointed out. But it sounds like it has so much potential and I'd really like you to keep going with it! I can absolutely see this fic shining! I hope this review has been helpful and I'll see you in chapter 2! :-)