Adaptational Heroism

A story is re-written and/or adapted. The main writer, either to suit the story better or because some would like to see the characters of the story act a little bit more heroic, decides to grant an otherwise generic, apathetic, or downright nasty character a bit of a lighter shade, or even lift them up to full hero status.

It could happen to either a formerly ambiguous character who decides to "take up the sword" and fight for the good guys, or one of the villains. Perhaps an enemy commander who in the original version was merely a one-dimensional obstacle for the heroes; yet in this adaptation, he would rather give up instead of fighting the heroes, lets them through, with small snippets of backstory showing that he secretly defied The Emperor's orders numerous times in order to save lives in the past. Or it could be bigger fish such as the enemy general; before, he was a ruthless person who would stop at nothing. In the adaptation he could turn out to be a Noble Demon whose main concern is leading his men to battle, avoiding the death of innocents if possible, and even though he does still fight the heroes he agrees to a fair fight and backs down once defeated, unlike in the original where he feigns defeat and stabs one of the heroes in the back.

Sometimes the changes made contribute positively to the story, showing formerly unseen heroic sides to characters. Other times it could come across as too deliberate to try and make a character more likeable, which of course can backfire. Maybe people liked the fact that the evil commander was reveling in his cruel deeds, his uninspired character being something amusing about him. On the other hand, The General could've been an Ensemble Darkhorse in the fanbase and a lot of fans were happy to see his character taken to new heights, rather than merely being portrayed as a stereotypical bad guy. Simply put, it can swing both ways.

In short, it's when a character is made more heroic or friendly in an adaptation.

This trope is the opposite of Adaptational Villainy. See also Historical Hero Upgrade, which is the same thing applied to a historical figure.

Anime and Manga

 * Fred Abberline in the first anime adaptation of Kuroshitsuji is possibly the most good-hearted and purely heroic character among the cast, while in the manga he seems to be developing into something more like an Anti-Hero willing to let his hands get dirty in the name of good. This is a case of Overtook the Manga. When the first anime was made, the manga had not yet given Abberline, a minor character, much in the way of a personality.
 * The same sort of process was responsible for how true character is dramatically different between the manga and the anime. In the anime he is good and kindly, but in the manga after a Reveal he becomes a major villain. In both cases he turns out to be  though.
 * Shampoo from Ranma ½ went from a manipulative bitch with no problems killing Akane in the manga (not unlike many of Ranma's female suitors) to a Friendly Rival to Akane with few qualms about her methods but considerably less killer instinct.
 * In the original manga of Yu-Gi-Oh!, Yugi's Super-Powered Alter Ego is at best a Sociopathic Hero and often seems more like a Super Powered Evil Side. However, in the anime adaptation, he's pretty much all-hero.
 * In the Sailor Moon manga, Nephrite is no better than the other three Generals serving Beryl. In the 1992 anime adaptation, he starts out as something of an Anti-Villain, falls for Usagi's friend Naru, and is redeemed by love; he dies from wounds suffered saving Naru from the other generals.

Fanfic

 * Any character who benefits from a case of Draco in Leather Pants can get upgraded all the way to a Byronic Hero or better.

Film

 * Bard in Peter Jackson's version of The Hobbit goes from a captain of the guard who is not important enough to be mentioned in the story before his big heroic deed of to a heroic freedom fighter whose big moment from the book was made considerably more impressively heroic and who then proceeds to even more action hero stuff not found in the book.