First Sister is one hell of a space adventure, no question about it. We get two stories in one for most of the book, in First Sister's perspective and that of soldier Lito. While their paths start off separately, we can assume from the start that they'll converge at some point, so I don't think that's particularly a spoiler. But at the start, these two couldn't be farther apart (physically and mentally speaking).
First Sister is, well, as the title suggests, a quasi-nun. The Premier Quasi-Nun, if you will. She's earned some favor among the powers that be, and as such, she's tasked with spying on new captain Saito Ren. Only, she finds that perhaps the Sisterhood isn't what she has thought it was, and maybe Saito Ren isn't the villain they're making her out to be.
Lito is having a rough time soldering on (pun absolutely intended) without his partner and incredibly close friend Hiro. Hiro's been missing since a big battle ensued, and Lito has basically been ordered to kill Hiro if he encounters them. I'm sure that will go well. The reader also gets little morsels of information from Hiro themselves, as they've left behind an audio recording for Lito to try to explain what has happened to them.
As you can see, the book is full of twists and excitement, and nothing is as it first appears. Therefore, I'll tell you no more. Best to go into this one not having any preconceived notions, just a basic understanding of who's who at the start. Then, you'll be in for quite the treat as the stories unfold and you begin to fall in love with the characters as I did.
There will be incredibly difficult choices, and a ton of character growth, and I loved watching it play out! My only minor qualms were a bit of a lull in the beginning-middle bit, but it was certainly not a huge deal considering I was already quite invested in the characters.
Bottom Line: I adored the characters and adventure in this wonderful first installment. Exciting and meaningful, The First Sister is a win all around.