Okay, so a few things first: I'm a cis person so take everything I say with that in mind. Though I have a lot to criticize, I don't want that to invalidate any trans persons experience who did feel understood, seen and/or well represented in this book.

Now that that is out of the way, I must say that I was very irritated by a lot of things in this book. For example it would've been completely avoidable to mention Leos dead name. If you're reading this review before reading the book, I'm kind of sorry to not put this as a spoiler, but not really. Because Leo's identity shouldn't have been treated as a little bit of a plot twist that you could've picked up on, but didn't have to - it isn't any kind of twist, it's simply his identity. I'm also confused why the word transgender only comes up a few times during the whole book, even though Kate supposedly did a lot of research on the internet, which would lead me to think that it's a word she would identify with. (And keep her from wording her surprise about Leo being trans as: "You're a girl"?!) Leo as well, but idk it kinda made sense that he doesn't and the word doesn't come up in his POV since he's really not happy about being trans and probably doesn't like thinking about it/identifying with it. Instead, we get several instances of wordings like "wants to be a girl", which is inaccurate given that Kate simply is a girl.
We don't really get a lot of explanation how she and her friends Essie and Felix got to the way they handle her identity. Kate says in the beginning that her friends know that she "wants to be a girl", but her friends never call her by her chosen name (which we only get to know way into the story) or by she/her pronouns, not even in private. Leo also doesn't try to adjust his perception of her after knowing, which I found strange since he should know how important that would be for Kate. (I guess being trans doesn't change that you might have to actively adjust the way you think about someone, but not even trying? Wild.) In fact, Kate is only actively called "Kate" to her face once in the entire book (and Leo uses her actual name only a few times in the last chapter).  

Still, I can't rate this book that low, because Leo and Kate did warm my heart. I liked them a lot and empathized with them so I couldn't bring myself to be too harsh. I just wished they were handled differently by their author. The writing style was also enjoyable, it was fast to read. I think I might try something else by the author, something that doesn't involve identities that can so easily be misrepresented.

Lisa Williamson is - to my knowledge - a cis woman who worked with trans youth for three years and yeah, this books feels like it was written by someone from the outside trying to look in (as someone who also isn't trans). I get that she had good intentions writing this, but I fear she failed a little and could've done a way better job. A part of the issues with this book might be due to it being published in 2015 (like the wording "wants to be a girl/boy" that was used and sometimes is still used to describe a trans persons experience to cis people - mostly when other cis people explain it). Back then the education on trans issues was way worse and the representation relied a lot on pain and struggle and was in general a little wonky. At least that's how I remember it. It's also not surprising that trans stories written by cis people gained traction earlier than own voices ones. So yeah, a bit of the book might be a "product of its time" kind of thing, but even in 2015 a lot of things could've been done a lot better. I'm not saying there aren't any people out there who had a very similar experience to Kate, but Kate's life is fictional, so the author could've chosen to tell it differently. 
I bought this book probably in 2016 when the edition I own came out and only now came around to reading it - I'm glad I read better done representation in the meantime.