Oreimo is a cute manga, which falls into the harem set-up common to Japanese anime and manga with Kyosuke being the main character surrounded by women while he navigates the strange world of anime fandom and encourages his sister to be herself. The cast falls into normal manga archetypes: Kyosuke is the cool but oblivious guy; Kirino is the cute girl who mistreats Kyosuke; Manami is the friendly, lovable girl and childhood friend who's totally into Kyosuke, but he doesn't know it; and Kuroneko is the scary girl and Kirino's frenemy. Of these characters, Manami is the one I felt the most attached to as the nerdy girl with a hardcore crush, even if the guy just doesn't see it. Unlike a lot of protagonists in this position, Kyosuke doesn't mistreat Manami beyond some light teasing. They are truly friends and we see them enjoy one another's company in a variety of situations, so he's not just using her for her book smarts. Overall, though, I really felt for all of the main characters. Even Kirino, who could be quite the b---h at times, was sympathetic in her fear of rejection for being herself.
Oreimo does brush up against some adult themes. I'd recommend it for 17+ readers due to the fan service scenes with Kirino (not nude but close), some of the ideas behind the Eroge (erotic video games) Kirino plays, and the pseudo-incest between Kyosuke and Kirino. That's right, I said, pseudo-incest. Apparently, big brothers and little sisters falling in love is a thing in Japan, and it's an idea that's toyed with in Oreimo as part comedy and part an excuse for fan service rather than it seriously being a love story. Kyosuke at one point even brings it up and is promptly struck down for his perverted thoughts. Also, a single “F-bomb” is dropped in the book, but it's unnecessary; it is pretty much the only time any of the characters swear and is used during a nerd debate, not in a sexual manner.
Moving away from possible incest and nerd debates, the art in Oreimo is solid. I'm not even going to try to pick apart the intricacies of manga art, but I did appreciate the amount of detail that went into the settings of Oreimo and the different outfits the characters wore, which felt right for each of their social clicks and was a welcome change from the constant sea of school uniforms. While the female characters have their cheesecake moments, it's not overdone and is usually there for another purpose, like setting up a joke.
Would I recommend Oreimo as someone's introduction to manga and anime? No, there are a few too many oddities that would be jarring to someone not familiar with the genre. But, for fans of the series, Oreimo is a cute book with a great set of characters that is worth the time and the possible ick factor from Kirino and Kyosuke. As Jemiah says in the back of the book, “Both the manga and anime series are so addictive, sweet, and wonderful at heart; if it’s a little sexy as well, what’s the harm? Kirino’s not our sister.”