Jacob Jackrabbit. Originally introduced as a big-mouthed but ultimately cowardly journalist, during Milk Trial arc he was suddenly changed into an evil, mastermind schemer who orchestrated all the events in order to usurp a position of power, and it was heavily implied that unchanged humans will be heavily oppressed during his regime. And then come Pigs arc, this plot element was abandoned entirely and Jacob is portrayed as well-meaning and conflicted leader of the colony, frustrated with some of the rat enforcers' methods of "maintaining the order".
Dottie Proctor, an experienced street reporter previously smart enough to work out the plot behind the Milk Trial arc from first principles, is hit hardest by this during the Pig arc, forced into a role that makes her feel more like a strawman caricature of a naive college student activist in order to act as a plot trigger while unaccountably failing to show the slightest interest in the big mystery of the pig disappearances. Her friendship with Walt was also inexplicably severed, and while she previously showed great deal of care and concern for him, during the Pig arc's epilogue she takes the message he left for her, puts it in a bottle of alcohol and angrily throws it in the water.
Dark Elf Drizz'l of 8-Bit Theater. In his very first appearance, he was stated to be stupider than Garland. By his second appearance, he'd already started drifting towards the Only Sane Man and Deadpan Snarker he would be for the rest of the comic. As with most such things in 8-Bit Theater, author Brian Clevinger did it because it would be funnier - having one completely sane character was funnier than one more complete idiot in a cast full of complete idiots.
Matt in Concession went from being a shy and polite virgin Christian boy who happened to be bisexual into a sex obsessed, diaper wearing, cross dressing, BDSM junkie who has sex with transvestites behind his boyfriends back and uses dark magic to fight priests who are trying to save the world.
Lord Sigfried von Damaske in Dominic Deegan. His original character was a Good Is Not Nice bully, but he began trying to improve his temper and people skills. Then, author Mookie got tired of people liking Sigfried, so he turned Sigfried into a racist and killed him. When people insisted on still liking Sigfried, he wrote a story arc where Sigfried had gone behind his friend Milov's back to sleep with Milov's girlfriend Jayden. After that, he wrote that all of Sigfried's earlier character development had been completely phony and done with the sole intention of getting into Jayden's pants.
In Red String, the author suddenly fell in love with a different love interest for the main character, Miharu, and had a problem in that she'd invested a lot of time in the original male lead, Kazuo, and how they were destined to be (as in the red string of the title). Near the end of the comic, Kazuo suddenly devolved into a creepy, awkward loser that "couldn't let go" - but as he rightfully points out, Makoto is a terrible person and in the very scene where he says this, Makoto has literally yanked Miharu away from him and yelled at her for continuing to associate with him. Naturally, as people still persisted in hating the Creator's Pet that was Makoto and still expected her to give Kazuo a fair shake, Gina portrayed Kazuo as forcing himself on Miharu as a last ditch effort to appeal to her. She wrote in an author's note on the original page that she did so to portray the scene as having "consequences" for him. He had never been portrayed as being this desperate and the act was clearly intended to contrast negatively to Makoto. The backlash from the original page (which showed Miharu actually wiping the kiss off) was so powerful that Gina eventually tried to redraw the page. Note that Makoto, despite constantly forcing himself on Miharu while she was dating Kazuo, is brushed off even by the woman he cheated on for Miharu as him just being a "flirty goofball." And in the final slapped together epilog that the author only wrote to get people to stop bugging her about what happened to Kazuo? Oh, she shows him as happy that Miharu has completely blown him off...when mere hours earlier, he was declaring that he still loved her. A bonus story for the omnibuses that was written in the final year of the comic when full Creator's Pet status had set in for Makoto was noted as portraying Kazuo in a much more negative and awkward personality than he had previously been portrayed as in the start of the comic. Meanwhile Makoto was shown as being a much nicer and kinder person than he actually was at that point in the comic. A lot of readers saw it as Gina trying to further retcon the characters to make the audience like Makoto more and Kazuo less.
