There is an archetype in anime and manga that I have started calling the “Emergency Heteronormative Character.”
At first, many comic creators didn’t know where their comics would end. Rose of Versailles is said to focus more on Marie Antoinette than Oscar. Kinnikuman famously started out as a superhero parody before morphing into a full-blown wrestling manga—and it’s all because its creator, Yudetamago, actually cared about the latter. A unique storyline in Yu-Gi-Oh! about a card game that forever changed its entire trajectory. I think the same thing happens with series where the relationships between characters are very important.
Some love triangles know exactly who the final girl will be, while others may not come to an answer right away (or ever). But I have also seen series where a specific character, usually a supporting character, seems to exist just in case, as if above them is a notice that says “Break the glass if necessary There must be heterosexual romance.”
I’ve never read Slam Dunk, but I’ve heard of Akagi Haruko: the female love interest of the main character, Sakuragi Hanamichi. She’s a pretty important character from the start (being the one who initiated Hanamichi’s entry into the basketball scene), and she’s even the focus of the anime’s hugely popular first ending. But over time, she recedes into the distance because the dynamic between the players themselves is what really draws people in.
The appeal of shounen sports series to shippers is also evident in this model. Whether it’s Prince of Tennis or Yowamushi Pedal, there often seems to be a female character who is like an anchor on the catabolic harbor, allowing the manga creator to fall back if needed. Even Saki has some of this energy in the early episodes. The character of Koutarou begins as the only male member of the mahjong club, serving as a potential male spectator stand-in to witness the girls in their recklessly apathetic glory. .
BL and yuri potential often bring many benefits in terms of interest in the series as mentioned. However, the Emergency Heterosexual Character can even exist in series that are quite heteronormative. In Rokudo’s Bad Girls, you have Tsuyukusa Mizue, the only girl in the series who hasn’t committed a crime. She is gentle and cute, always worried about the main character Rokudo seemingly turning to the dark side. And while the anime takes place in an accelerated timeline, the beginning of the manga makes it pretty clear that she could have been Rokudo’s “saving grace” if the series had played out a little differently.
Emergency catabolic characters do not. aren’t automatically bland and they can be fun and charming in their own way. That said, they often feel like the product of an author hedging their bets, and they often shine less because they’re simply not intended to get much attention. I also have to wonder if these characters exist to some extent in case the title needs a quick romantic ending if things need to end quickly. However, as we move away from an era where nice and neat heterosexual relationships were considered necessary, it’s possible that the archetype will have to evolve into something else entirely.