Since the first time I met Nico di Angelo in The Titan's Curse 10 years ago, I have LOVED him. I became very attached to him and even more while witnessing his character development throughout the camp half-blood chronicles. I probably feel so drawn to Nico as I was going through hard times myself. What a joy it was to finally read a book entirely dedicated to Nico. It fills my heart with joy, peace and relief to get to finally see him heal and find peace after all this time -- just like I did. So, as Hades said, Nico deserves everything.

I prefer to start with the things that bothered me a little so I can end the review on a positive note. Honestly, this co-author thing didn't work great because I thought the voices didn't merge well: you can feel who wrote what. And what happens when you let your characters be used by someone who hasn't spent the same amount of time with them, or who hasn't being working on them for decades, is that you get some characters such as Mr D. who don't feel like their "authentic" selves; they don't feel like and act the way they've been for the rest of the series. It also felt like the authors suddenly at one point remembered that Hazel is Nico's half-sister, but there wasn't enough of her in Nico's story, like he barely thought of his half-sister. Maybe it's because Hazel is one of my favourite characters, but it felt wrong to not give her an important place in a Nico di Angelo book.
Now, the thing that bothered me the most was how a few times, it felt a bit like fanfiction. Like this was a fandom version of Nico, Will and Solangelo. I'm not sure how to explain it though and that feeling actually left after hitting like the 50-60% mark.

As for the rest, I simply enjoyed it. Loved it. It's Nico and I love this fictional character so so much idk man he's everything to me.
Although reading about Nico's trauma again, his nightmares and his doubts hurt, I loved reading Nico's flashback tartarus journey (those dark pages in the book).

I thought Nyx had great potential as the villain and made sense in a story about a child of Hades, filled with a lot of darkness within him.

And I enjoyed the talk regarding darkness vs. light and realising and embracing both at the same time. That was good for Will's character development too. About the whole miscommunication stuff.... I know it's annoying, frustrating, but they're kids, and queer kids (same as how it is in Ophelia's After All it was frustrating but understandable cause they're children and queer). They're finding themselves, learning, growing up. Both Nico and Will grew a lot out of this journey, especially in regards to their relationship.
I do have mixed feelings about how Will was treated because: I kind of thought he wasn't enough developed like yes he's the son of Apollo, he's a literal sunshine, but somehow I still felt like I didn't really know his personality? Like his personality traits are being a child of Apollo, being Nico's boyfriend, being an extrovert and? (Then again I have yet to finish The Trials of Apollo series so maybe we learn more about him there). I also felt soooo bad for Will because he was really trying his best in the most hostile environment that can exist for him while feeling useless at the same time (he was not useless). This was very brave of him to not give up. So to add to his personality traits: he's brave and very loving. I'm glad we got Will's point of view, it added to the storytelling and showed us how much love Will has for Nico.

The plot twist towards the end with Nyx was a bit... weird but at the same time made sense?
ALSO, oh my god chapter 48!! I'm not someone who cries or even has one tear while reading but seeing that little reunion with the di Angelo family made me fight tears stuck in my eyes!!! Bianca!!!! Mamma Maria. Nico is so loved! This was very much needed.

Overall, as it's a middle grade book, I'm glad queer kids will have this story. And I appreciate R. Riordan's effort in inviting a queer co-author to tell Nico's story, so we can have a queer author telling stories about queer kids.
The Sun and the Star is Nico's quest yes but it's mostly him facing his trauma, learning to let go of his pain while not ignoring his past, and learning to let people in, let people love him for who he is. It's about Will learning to not have so much prejudice against something that is so different from what he knows and learning to love someone for all they are and have. And it's about romantic love, friendship love, family love, and self-love.
Some quotes were quite emotional lol but I can't remember the pages and where to find them again so I'll end with this one, still fresh in my mind and which, although used here quite literally, I find it very poetic. It is from Will's point of view:
"Will had heard love described in so many dramatic, bizarre ways over the years, but no one had described it like this: it's like drifting down a river of pain and knowing you are safe."