So, a recent email plus a few older essays got me thinking about something, and that something is the continuum of intimacy in relationships, especially in Japanese literature (including anime and manga).
First, the hypothesis: I have the textually based impression that Japanese culture does not set sexuality apart from other kinds of relational intimacy, but rather considers it one integrated aspect or thread of intimacy as a whole. The continuum of intimacy runs from mild to intense, and each aspect or thread runs the entire length of this continuum. Further, the aspects of intimacy, which seem to include sex, fighting and family, can slide into each other without warning--the boundaries between them seem very fluid. One aspect does not presuppose any other, but the existence of intimacy in general may contain any or all aspects running parallel to each other. Sexuality is not considered a different kind or level of intimacy--it is not privileged.
Secondary hypothesis: US culture does privilege sexuality. It is considered the culmination of intimacy, the most intense sort available. Romantic lore suggests that friendship is a lesser intimacy, and that rivalry is not intimacy at all though undying enmity may be. Personally, I blame this on the 19th century, and the valorization of the forbidden, but that's another essay.
Anyway. On to the examples that incline me to think this.
One set of examples has to do with siblings. The sibling bond, in every text I have viewed yet, is unbreakable, even when they're fighting. Trigun (anime): If Knives can't convince Vash to agree and stay with him, he has to kill him; that's the only action that seems to have the same intensity. Samurai Deeper Kyou (anime): While the Sanada brothers are dueling, the elder feels impelled to explain to and instruct the younger; it could be argued that this contributes to his loss, but he does it anyway. Digimon Frontier: Kouichi is drawn back out of brainwashing by facing Kouji, and, once he is, Kouji promptly abandons his rival for his brother, except when their evolutions demand he and Takuya be the two who fight (more about rivals in a bit). In the absence of a strong maternal figure, especially, the sibling bond is often joined by a sexual bond. Card Captor Sakura: Touya is determined both to nurture his sister and to keep any possible romantic suitor away from her, to keep her for himself though he does not express sexuality toward her on his own account. Mai-HiME (anime): Takumi has explicitly sexual feelings for his sister, and his importance to her is paralleled with the bond between the only explicitly established romantic couple. Utena: Touga's sister, Nanami, is frankly obsessed with her caretaker/sibling, and the question of whether Anthy is princess or witch repeats the confusion and cross-over of her roles as lover or sister to Dios/Akio.
One of the things this theme rests on and appeals to is the importance of family as a whole, of blood heritage, and of the mother. The family is the most basic and original in-group. Spiritual inheritance or inheritance of character is assumed to operate through blood connection, to the extent that shared blood plus shared name traditionally equates to shared self, thus the traditions of renaming a child after a dead relative (eg Nuriko in Fushigi Yuugi, and a dozen others I've never watched myself). Thus, siblings have a very intense standing assumption of intimacy. The mother, according to the patterns highlighted by soap opera, has first claim on her son; the wife's claim comes second. According to the patterns of folklore, the mother is the center of the family. The mother is the single most likely figure to to express every aspect of intimacy at maximum intensity (eg the mother and daughter Subaru deals with in the X manga). Hence, the sibling that substitutes for the mother, inheriting this primacy, often becomes the primary romantic object as well. Siblings can, however, add the sexual aspect even when a mother is present, as do Setsuna and Sara in the Angel Sanctuary manga. On the other hand, some mother figures are powerful enough to hold the primary focus of their children even after they disappear; Ed and Al of Fullmetal Alchemist are a good example of this. These two examples illustrate that this is not some kind of infallible equation, but a flexible theme. It depends on the direction and style of the story, as well as the presence or absence of particular key archetypes.
Moving on, there's the example set of rivals. The term "rival" encodes both competition and similar strength. Any show in which fighting is a main element is almost sure to feature meditations on the importance of having a good rival, the uplifting experience of fighting a good rival, and the spiritual bond between good rivals. A rival is not, note, just someone the hero fights with; a rival is a very specific kind of opponent. A rival participates in the same arena of competition and acts according to the same rules and standards and ethics as the hero. To call someone a rival indicates a respect for their ability. To call someone a rival when they are, in fact, notably superior is extremely gauche, and forms a stock character-joke. A rival is special, and rival pairs measure themselves according to each other, thus the convention that it is bad form to hold back with one's rival if you also happen to be friends. Rival express their appreciation and understanding of each other by fighting. Naruto and Sasuke of Naruto. Touya and Shindo of Hikaru no Go. Seto and Yuugi of Yuugi-ou. Yuusuke of Yu Yu Hakusho and, well, at least one character in every arc, some of whom he keeps for his own side afterwards (Kuwabara, Hiei). Kyou and Yukimura of Samurai Deeper Kyou. Atobe and Tezuka of Prince of Tennis (followed by just about all the other captains and Tezuka, too). Half the cast of Initial D with any other half of the cast. The focus of rivals on each other easily matches the focus of lovers, in intensity and exclusion of all other considerations. In many ways, CLAMP's concept of the dark self strikes me as the ultimate expression of the rival--actually a part of the person in question.
I do not, actually, think that rival and lover have as strong a tendency to overlap as sibling and lover. The traditional Japanese concept of lovers encodes imbalance, rather than equality, very strongly. The intensity of those two aspects of intimacy is equally strong, but they run more distinctly parallel than the aspects of sibling-lover or sibling-rival, which cross more fluidly. The only time rival and lover seem to cross easily is when one of the rivals is significantly in advance of the other in age/expertise--Gon and Hisoka in HunterXHunter, for instance. This cast iron supposition accounts for the overwhelming tendency, in Japanese fanart and fanfic to de-power one of the participants when crossing rival-lover. Which seems to me to rather miss the point of rivals, but there it is. I am more than half convinced that this tendency is the root of why shounen mangaka consistently make their more soft-spoken and gentle characters taller and their more aggressively masculine characters shorter--it means that the fans who subscribe unilaterally to the height=dominance formula will be forced to so alter the characters' personalities that they become, effectively, totally dissociated from the source text (eg Hiei and Kurama). US fen are more likely to keep both rivals at full power, since it is exactly the intensity of the tension between them that inclines US fen to leap to a sexual conclusion, sexuality being, in this culture, assumed to be the highest-energy intimacy state.
Actually, what I see US fen doing is circumventing the standard model of intimacy somewhat. Romantic lore says that rivalry is not a form of intimacy (being on the same side, yes, but much less so for being on opposite sides). Enmity, however, is an intense form of interaction, which makes it relatively simple for US fen to map enmity onto what is, theoretically, a totally separate form of intensity: sexual intimacy. This already-established maneuver has, I think, gotten a big boost from anime and manga imports, seeing as those do understand rivalry as intimacy already.
The categories of family and rival slip into each other not infrequently, much the way the categories of family and lover do for the siblings mentioned above. Yuki and Kyou, in Fruits Basket. Shuusuke and Yuuta in Prince of Tennis. In these cases, the rival-ness often becomes more one-way, because the common understanding of family and siblings encodes inequality, as well, though not as drastically as the understanding of lovers. The elder sibling is the caretaker and protector (eg Yamato and Takeru in Digimon Adventure, Mai and Takumi in Mai-HiME, Hokuto and Subaru in Tokyo Babylon); when this is impossible, or when it breaks down, it is a tragedy (eg Kouichi and Kouji). Thus the assumption that one family member will be consistently chasing after the other, if they are also rivals.
My favorite example of all these aspects running hard alongside each other is Tenipuri's Fuji brothers. They're siblings, with the elder-protector dynamic; they're rivals in that Yuuta's goal is to match/exceed his brother in the game they both play and Shuusuke accepts this; while they are not (probably) lovers, Shuusuke displays all the hallmarks of a brother complex, a la Touya of CCS--wanting to keep all competitors for attention away from his sibling. None of these aspects is an advance in intimacy over any other, rather they are alternate expressions of the same degree of intimacy.