Mon Dieu, please stop with the torture! Alex is a Mary Sue, don't you see the ratings? That is very bad and your character Alex, if you read very, very carefully is just a fancier way of saying it is you. Or what you want to be like. I am sorry but this story is horrible. I do like the way you write so I will never say to stop doing that, but please make a character that has a bit more background in magic and one that does not come from America.
Now some advice on this character. To have an OC is a lot of fun because I find that it is unique; however she/he needs flaws and a decent character and Alex has neither. You need to develop Alex's character fully so that we can like her... which means to stop playing the hero and not show up randomly into the Harry Potter universe. Have Alex go to an American witchcraft and wizardry school and for some reason her family decides to move to Britain. Not only will that make Alex's character more believable but it will also prevent her from becoming an annoying fangirl's fantasy and to prevent her from being a Mary Sue. The way I was taught in an English class was to introduce your character as if you really know them. Due to the fact that she is not developed at all, Alex seems as if you just wrote out the story and added the character into the mix. You need to really think hard on how to introduce these characters and to put much thought into them. To me that means with each character trait or personality that I get to colour them in a bit. Alex is black and grey and does not have colour to her at all.
This story really annoyed me with how Alex seems to take all the limelight and within a fanfiction that is never a good thing. Allow the canon characters such as Remus, James, Sirius, and Peter shine through as well. Besides the plot doesn't have twists and turns that would normally hold my attention and keep me reading it. It just seems as if you decided to write out this story and do not care at all for what you write. You should also space out more, that enter key is there for a reason and it keeps things from getting cramped and all.
I highly advise you to take a look at Alex's character and really define her more. She is very clingy to the canon characters and if this was ever a real situation in the book then I doubt very much that Sirius, Remus, and James would like to be friends with her. Besides Alex's character is really annoying and I hated her since the first chapter. Make sure that she has a personality and has some flaws... that will put more colour to her character and will make her more enjoyable to read. Also in future, you should really did down her appearance. Yes I said that and I will tell you why: To have Alex as the main character means that you take away from the canon. Of course I like OC's in fiction but only true fans know how to blend them in seamlessly into the story. They know when to prevent their OC from becoming a Mary Sue and those are two things that you have failed at doing.
If you want to continue the story fine but don't expect any nice comments for it. Have a good day and good luck with your story.