Hi there! Here I am, with your requested review. I have to say, when I read the summary for your story, I was definitely intrigued--many NextGen stories seem to be of the "day in the life of a teen" variety, with the main focus on relationships and Quidditch. It's always more interesting to read one that goes beyond that, especially when there's an element of mystery in the plot!
A couple of the things you mentioned in your areas of concern were flow and characterization. I think the flow was generally good, though I think it might have been nice to break up the chapter once or twice. The train, the boat ride across the lake, the Sorting, and being shown to the Ravenclaw common room is a lot of ground to cover in one long scene. So, for example, you could have had a scene break at the end of the conversation on the train, and skipped straight to the boat ride. It just makes the experience of reading a little more varied and dynamic.
As for characterization, I really like Arianthe! I think it's great to see a Malfoy being Sorted into a house other than Slytherin. You wrote her perfectly for her age, which can be difficult to do. It was good to show her inner conflict as she arrived at school and found out more about her family's past; her refusal to believe it was really in-character.
My only suggestion in this area would be that I would have liked to have a bit more explanation of Arianthe's background. I didn't get a good sense of the Malfoy family dynamics--there were hints at it, like Arianthe's puppy-dog-eyes, and her description of Scorpius and Selene, but I felt like we kind of hovered on the surface and never really delved much deeper into it. Draco seemed much more doting and fatherly than I would have expected, so it would have been interesting to hear how his canon personality has changed since he's had children. Also, I don't think you mentioned in this chapter how much older Scorpius and Selene are than Arianthe, which is something I think readers would naturally question. It also would have been interesting to read more about the dynamics between the Potters/Weasleys and the Malfoys. It would be good, I think, to explain why they're all friends (or at least sort of friends), despite the history between their families. And on that note, while I found her discovery of her family's reputation to be one of the best parts of this chapter, I have to wonder how she didn't find anything out before now--surely, wouldn't she have gotten some inkling of it before? She might have heard her brother and sister talking about it, or noticed people's reactions to her father when they were out in public. Addressing that could make her revelation all the more significant.
Introducing a character well is a really important part of a first chapter, and it can be hard to hit the right balance. One other thing I noticed is that you seem to gravitate more towards writing dialogue than description (and I do that, too, so I understand!), which can be more problematic in first chapters than in others. It's hard to feel like you're getting to know a character when most of it is dialogue, and a lot of it is dialogue from characters other than the protagonist. I think this chapter could be strengthened by the inclusion of some more background information, not just about Arianthe, but about her family and their place at Hogwarts. Maybe you get to explaining that in later chapters, but I really think it would be good to have at least some of it in this very first chapter, to answer the questions that I'm sure are on many readers' minds. If it's an issue of spoilers, you can include some cryptic statements, just so the readers know that you haven't neglected to consider it.
One other thing I noticed is that your dialogue isn't formatted properly in all cases. This formatting is perfect: "I have a brother too," I said, glancing back at Morgan curiously. You've ended the quotation with a comma/half-stop. In many other places, though, you ended it with a period/full-stop. I actually wrote a guide to formatting dialogue that's in the grammar guidelines subforum in the Writer's Resources area over on the forums. If you aren't sure about how to format your dialogue, I think you might find it helpful.
So, all in all, I think this chapter was really intriguing, and I liked your protagonist. The only thing I found myself wishing for was more explanation--which, if you think about it, isn't necessarily a bad thing. Your story is great; I'm just curious and want to hear more about your characters! :)
Author's Response: Holycraplongreview.
Helpful, though. I won't address every single paragraph, which isn't doing this review the justice it deserves, but I will say thank you so much for taking the time to do this for little ol' me, and also, about the formatting thing - I acutally did read that a while back :) looks like I need to brush up on my skills, though. It was very helpful :)