Glorifying a character
The concept of glorification is similar to the concept of a message, but more nebulous. The amount of glory you give to a character dictates "the story's" opinion on how cool or good the character is.
Messages in fiction
This is important even if you're not trying to send any messages with your story. If you've done a good job creating lovable characters, readers will likely get very attached to their favorite ones and be upset when those characters are de-glorified. The Doki Doki Literature Club fanbase is an excellent example of this phenomenon. That's because Doki Doki Literature Club is an excellent example of how to create lovable characters. And no it's not because it's a romance game (it isn't) and the girls are specifically designed to look as cute as possible (they are).
Doki Doki Literature Club
So I think there are three main things that contribute to glorification: competence, agency, and morality. Morality is usually the brunt of how people decide which characters they *want* to see glorified. But if you're late enough in the work that the characters are well-established, or especially if you're making a sequel or fanfic for an existing work, having someone do something uncharacteristically reprehensible is likely to come off as slandering a likable character rather than establishing an unlikable one. Arcs are one thing, but suddenly forgetting or changing a character's traits out of nowhere is another.
Basically, it's important to pay attention to how much glory you're giving each character. If you give a character more than they deserve, readers who dislike the legitimate flaws of that character will get annoyed, and if you don't give them enough, readers who like the legitimate virtues of that character will get offended.
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