The characters don’t grow very much. Some characters get tiny little bits of growth, but nothing changes very much.
I get the sense that the author was just presenting these characters for the delectation of doujinshi mangaka. The author did not want to write a romance between a 14-year-old girl (Chaika) and an 18-year-old boy (Toru), but the ending suggests nothing so much as a too-good-to-be-true romance between the two main characters. There are going to be plenty of doujinshi stories written about the exact details of that romance. This is not a colossal tragedy, but it does illuminate something about the kind of fiction I dislike.
If you’re going to design characters for porn stories, you should have the honesty to put them into an explicit romantic relationship in your story. If the writer wants Chaika and Toru to be lovers or get married, he should write the story of exactly how they enter romantic commitment. He shouldn’t leave it hanging for endless sequels.
Comic books and pulp stories are sold to make a profit and it’s not surprising that their characters never grow. The action-adventure pulp formula is pretty predictable, and cannot tolerate a lot of character growth.
However, after many years of consuming pointless, unrealistic “action-adventure” stories – it’s beginning to stick in my craw. I want a better class of story.
Western pulp writers are mostly writing fantasies of violence – swordfights, explosions, shootouts – just like they’ve been writing for more than 100 years.
In the case of Japanese writers, they are writing for Japanese audiences, and teenage romance stories might push the limits of legality. Japan’s government is not going to hunt down doujinshi mangaka who write stories about Chaika giving her virginity to Toru, but such a story might very well be illegal in Western countries. It’s entirely possible that the series creator resents such legal restrictions and refuses to write the story he really wants (e.g. underage marriage) because Western governments and international censors might prosecute him for it.