Koi Kaze

After his girlfriend dumps him for someone else, 27 year old Koshiro has a chance encounter in the subway with Nanoka, a 15-year-old high school girl. He later meets her again at the entrance to an amusement park and since he has just been given two free tickets to it, he invites her to go with him. Each being rather depressed over the end of a relationship, they immediately feel some emotional connection across their age difference as the afternoon progresses.

On leaving the park, they suddenly discover that they are brother and sister when they both recognize and greet their father as he's arriving. This comes as quite a shock, since they haven't seen each other since their parents got a divorce with split custody some 10 years ago. Moreover, Nanoka is now moving in with her father and brother since their house is much closer to the high school she's attending. Against their better judgment, the first spark of mutual attraction slowly ignites into something deeper and more powerful as time progresses, especially since Nanoka develops a huge big brother complex and she is not deterred in the least by Koshiro's boorish and unsociable nature.

Koi Kaze started out as a manga series which then got adapted into an anime series in 2004 and immediately received a lot of attention for its harrowingly realistic examination of sibling incest. The painful and guilt-ridden romance between Koshiro and Nanoka is credibly full of angst and Belligerent Sexual Tension as the two struggle against their desires, refreshingly devoid of the blithe disregard for social mores and Fan Service most Shonen series on similar subjects tend to have.

Tropes used in Koi Kaze include:

Arranged Marriage: Koshiro works in an omiai (dating arrangement) office. He even faces the possibility of having one of these dates at some point. He refuses.
Adaptation Distillation: The anime is very faithful to the manga, but it compresses the two-year period encompassed in the manga (three springs) to one year (two springs). Also, oddly, the events of the manga's second chapter were made into an unaired episode 8 of the anime, later restored to the DVD release.
Bittersweet Ending: At the end of the anime, Koshiro and Nanoka are together and happy, but there is unmistakable symbolism that, at the very least, they are still facing a very difficult time to come. The makers of the anime obviously wanted to make a point here.
- The manga has some foreboding symbolism in the chapters leading up to it, but ends on a more positive tone, with Koshiro and Nanoka contemplating the inevitable challenges ahead (symbolized by the park where they "met" on the verge of closing) with open eyes and resolving to stick things out anyway.
Bottle Fairy: Chidori, who once partied so hard that she was calling herself "Midori" as she pressed more booze on Odagiri to get him drunk with her.
- In addition to drinking hard, however, she's also rather hard-bitten and sometimes gets rather mean when she's drunk.
Brother-Sister Incest: Goes without saying. One of the best deconstructions of the trope around.
- Something of a reconstruction too, however: could a brother and sister romance really happen, in view of what we know about the Westermarck Effect? Yes it could, at least if the brother and sister happened to be, say, separated at a very early age due to a divorce. Somebody apparently Did Do the Research.
- There's also Genetic Sexual Attraction to explain why they have such strong feelings towards each other.
Cherry Blossoms: You can't have a Japanese romance without 'em.
Christmas Cake: Koshiro's family and coworkers wonder why a guy his age isn't married yet.
Comic Book Time: Averted; Nanoka clearly grows older as time passes.
- Koshiro's last words in both the anime and manga, said with an emotional expression that contrasts jarringly with his numbness at the series's beginning: "Love you.... I love you."
Deliberately Bad Example: Odagiri openly lusts for high-school-aged girls and sure wishes he could be in Koshiro's situation with Nanoka--which, of course, is exactly why he's not; he's mostly there just to remind us Koshiro is nothing like him and never asked for this whole situation to be foisted on him in the first place.
Driven to Suicide: Nanoka at one point suggests committing lovers' suicide, but Koshiro objects. It seemed intended not to be taken too seriously, but her reason that it would much easier than trying to figure out their relationship implies she was at least half-serious.
Earn Your Happy Ending: What Koshiro and Nanoka are still striving to do by the end.
First Kiss: A bit ironic when Nanoka initiates a serious kiss with Koshiro and then she yells, "Aahhh! It's gone! My first kiss!"
Hidden Heart of Gold: What Nanoka thinks Koshiro has. She's not wrong, at least where she's concerned.
Jerkass: Koshiro for the most part, until he and Nanoka decide to hell with everyone else, they will be together.
Love Bubbles
Luminescent Blush
May-December Romance: Aside from being her brother, he's also twelve years older than she is.
No Periods, Period: Subverted: Nanoka has her first period in three months and suffers some severe cramps from it.
Not Blood Siblings: Cruelly averted; though the series offers a few hints here and there that it's going to pull this particular cop-out and tell us Koshiro is actually adopted, it ultimately pulls the rug out from under everyone wishing this were the case.
Solomon Divorce: Nanoka and Koshiro are apparently only reunited for overall convenience.
Surprise Incest: Although they do find out rather quickly that they are related.
Together Umbrella: Turns up in a couple of episodes, especially during the rainy season.
Visible Sigh: Also makes an appearance or two. One humorous variation had a cloud forming over Nanoka's head and exploding with an audible pop when a boy mentioned rumors he'd heard that she had a brother complex.