Joni from Boy Meets Boy is shown as a good, somewhat tomboyish straight girl and friend of gay teens Paul and Tony. But then she hooks up with dumb jock Chuck, who is really most likely dating her out of spite for a previous girl who spurned his advances, and things go downhill from there. Although her friends try to like Chuck, it's no good and then they don't try very hard to hide how much he annoys them. Even so, this doesn't excuse Joni for giving all of them (except Chuck) the silent treatment, not returning calls, or dropping completely out of their lives after about a decade of friendship. To her credit, she probably goes back to making nice with Paul and Tony in the end.
In El Goonish Shive, Ellen is by no means a bad person (her attempts at being the Evil Twin didn't work out very well), but even though she was created as a duplicate of the female Elliot, she is notably more insensitive and self-centered than he is. The reason for the change in personality can probably partially be blamed on the traumatic circumstances of her creation, partially on alter-Ellen's dream life, and partially on an intentional desire to make herself distinct from her "twin brother."
Due to sleep deprivation, Jade Harley has gotten a lot grumpier - although it should be noted that she's only like that to the trolls, who have been picking on her for her entire life (with the exception of Tavros). She got better later on, though.
According to Aranea, Cronus wasn't always as big a Jerkass as he later became, but he was never exactly a nice guy, either. Apparently, the revelation that magic didn't exist - something he'd based a good deal of his life around - caused a Heroic BSoD leading to this trope.
And now, Aranea herself combines this with Face–Heel Turn, becoming not only more sadistic and arrogant but also unbelievablysmug.
DiDi originally appeared to be sweetly nice if ditzy, but her desperate desire for an orgasm eventually drove her to increasingly erratic and sometimes bitchy behaviour. Some readers, looking at her earlier casual, careless treatment of her numerous boyfriends (and her later insanely unthinking treatment of the frustrated Zii), suspect she was a bit of a Bitch in Sheep's Clothing all along, but it might be more accurate to say that her ditziness is actually just extreme social and emotional incompetence.
Sonya also initially came across as The Ditz and none too bright, and rather the victim of Zii’s Casanova tendencies. However, she soon discovered a quite dangerous love of Drama and even danger and demonstrated increasing levels of low cunning, and while her Stalker with a Crush behavior could be seen as Zii’s follies coming home to roost, Sonya proved quite willing to use anyone to get her way.
Tracy initially looked like lead character Gary’s born geeky soulmate. However, she then started coming across as ridiculously pushy and sexually manipulative, bouncing Gary into sexual activities that he never really wanted and that looked borderline non-consensual. It's telling that even Extreme Doormat Gary eventually got fed up and washed his hands of her. However, it’s questionable whether this was Character Development or whether she was probably like that all along.
Problem Sleuth: Mobster Kingpin's transformation into Demonhead Mobster Kingpin had made him much more evil and powerful. Then he became Dark Matter Mobster Kingpin...
Sandra big time during her Face–Heel Turn. It takes a while before she reverts back to a nicer persona.
Wally, post-timeskip, acts a lot more insensitive towards his friends (see Innocently Insensitive), and has no qualms about saying he is willing to destroy Sandra in front of her former friends. Said friends include his current girlfriend, Crystal, who he knows was Sandra's closest friend.
When Stephen first approaches Bryce, Bryce is receptive to the attention and happy to join the friend circle between him and Gray. But the longer that Bryce stays at Stephen's side, the more that his inferiority complex gets the better of him, until he's mutated into a full-blown jerkass.
Wesley is introduced in Season 1 as a minor character who was complicit in the bullying that Stephen experienced- however, he wasn't an outright bully himself. This changes when he makes another appearance much later in Season 3: not only is it revealed that his hobby is falsifying pictures to spread rumours about innocent people, but- as revenge against Gray for grievances that aren't entirely Gray's fault- he uses said hobby to fake a picture of the comatose Stephen to traumatise Gray.
time to sabotage CK