Oh the number of ways this could've gone wrong. Koi Kaze is about incest; there's really no more basic statement about the show to make. Of course anime and incest have always been close relatives of one another, but after the third romantic comedy featuring horny young sisters grown up and still looking to bathe with big brother, one starts to suspect the writers haven't actually thought too closely about the weird psychological grey area they're jumping into. But Koi Kaze not only refuses to dodge reproach in the usual “they're not actually blood-related” way, but it embraces every ounce of biological conflicts and social stigma, at which point it's not too surprising it's actually a full-on drama and not a slapstick lovefest. Does that make it good on it's own? Of course not; that credit goes author Yoshida Motoi, director Omori Takahiro, and the rest of the team. They dodged more bullets than I would've possibly expected with this one.
You can tell they were conscious of how fundamentally unlikely a 27-year-old brother and 15-year-old sister would be to fall in love romantically by how they covered their bases in the setup. First, they weren't living together for most of their lives. When Koshiro and Nanoka's parents divorced, Koshiro stuck with his father, keeping the last name Saeki, while Nanoka went off with her mother and changed her name back to Kohinata. Since this was when Nanoka was a baby and Koshiro was a teenager they were in the age window to dodge the Westermarck effect, pretty much the predominant biological impetus against sleeping with people you knew from ages one to six, but it also makes sense that they would forget each others' last names, or even each others' existence in the hubbub of day-to-day life growing up, going to school, getting a job, getting a girlfriend, and so on.
Yet they also crafted the right circumstances to bring them together in that way. A series of coincidences lead the two of them to enjoy a day at the amusement park, not yet knowing that they're siblings who would be living together starting that evening. Koshiro finally lets himself cry over his recent breakup, and Nanoka gives him the chance to let it all out. Once they start living together she is affectionate towards her new big brother, but he acts cruel out of anxiety towards his feelings for her, pushing her away. Once he starts to get himself back under control he goes back to treating her more kindly, looking out for her and helping her with all the things a big brother would, boomeranging her anger towards him so fast it would make Keima from The World God Only Knows proud. And then living together does the rest of the work.
I will say that while Koshiro's anxiety and treatment of her makes some sense in the context of liking his sister, and especially in the context of being an anime character in a burgeoning romance, his attitude was over-the-top for the first half of the show. He shoots her down coldly every time she asks him even the most inane question, or shows him the slightest bit of affection. And then in his personal monologues, he searches for justification, from “what's that brat doing” to “it's normal to masturbate to the image of your little sister, right?” The portrayal of Nanoka worked a lot better during that time, partially because her attitude as a sister and a friend hit a nice middle ground of affection without being Koshiro-centric. A good amount of credit there belongs to the softer art style and Nakamura Yuki's voice work, which both softened the image of Nanoka and made her feel more like the average high-school girl. Being mild-mannered and a secondary main character to Koshiro, our insight into her thoughts is a bit more limited though, so it might become a bit of a surprise when she kicks the romantic side of things off around the midway turn.
And that half, which we were of course promised from the outset as the premise of Koi Kaze, is where things shine. Koshiro tries to escape, knowing that it's his responsibility as the adult to keep Nanoka from making a terrible decision, but he folds without even noticing, looking for new apartments in the morning and then forgetting by night. Their words become less restrained in their content, but interestingly enough they become more controlled in their emotion, and the drama starts to come more from the reality of their situation than from shouting matches and inner turmoil. His close coworker Chidori had always been a great addition to the show, a platonic female friend who teases him over his sister but ultimately listens to him over a drink every time, but with things developing between Koshiro and Nanoka she captures the stigma of incest perfectly, trying to reconcile anger at Koshiro with her fear that their lives will fall apart, and looking for any reason to understand their situation. Her last line to him is telling: “I won't tell you good luck, but take care of yourself.”
Ultimately their decisions have consequences. That seems obvious to say, but with the romance appearing in the second half it would've been easy to leave the ending wide open, with no implications of reproach. The ending is certainly open enough to imagine they find happiness, but through Chidori and their closed approach to the outside world in the last few episodes, captured perfectly with shots of Koshiro looking like a beggar in the supermarket as people keep their kids away, we know that they're in for a fight, as lovers who are siblings twelve years apart. The palate once again adds a softness to this ending, a rose-colored tint that makes us feel like they'll survive somehow. But we predominantly see spaces like his dirty apartment, the alley behind his workplace, or a pier lined with tetrapods where the two of them wonder if it's better to run away or even die, spaces that are rooted in the detail and inconvenience of the real world. In these spaces, in this world of Koi Kaze watching the mismatched siblings being intimate and going beyond the boundaries of what they know how to responsibly do or say is painful, even through the rose-colored lens.