It doesn't seem to have stuck. Although the fact that she has to vanish Klaus to "remove temptation" kinda makes it dubious whether it had any chance of sticking.
El Goonish Shive: After she was created Ellen tried to be a villain but wasn't very good at it. She eventually became a hero.
Hedge, Guineas and Vlad(ia) also have one almost immediately after Damien's death.
Damien was a VeryBadBoss. All three of them were either misled or directly threatened into being serving him. Also the reason Grace was on the run in the first place. She just refused the Face–Heel Turn in the first place.
Abraham after Nanase talked him out of killing Ellen.
Angel Moxie: Tristan. She (yes, she) turned good because the Big Bad screwed her over, though it's obvious that Tristan wasn't exactly evil in the first place, just easily manipulated. In an unusual turn, it's revealed she was one of the Legendary Heroes from the start.
Despite being pretty unlucky at love, Torg from Sluggy Freelance seems to have this effect on the ladies:
He turned Aylee from an amoral, man-eating alien into a rather pleasant friend and employee who only occasionally ate human flesh.
He was the first person to befriend the demon Mosp since her transformation, leading her to rebel against Lord Horribus.
It takes some time and some pretty hefty promises on his part, but Torg eventually gets Ax-Crazy, love-sick assassin Oasis to give up killing people ... well, unless she decides they're bad guys. Still, for her, being an ultra-violentvigilante is a huge step forward.
Ahmad in Harry Potter Comics undergoes this after truly realizing what the results of he and his brothers' jihad has been.
Also, Belkar's pretending to have undergone a face turn after his brush with death. He's still the Token Evil Teammate, but acting good keeps the authorities off his back and lets him Mind Screw the rest of the party. While Roy and Haley aren't fooled, suspecting it's a scam of some sort, they don't care either since Roy knows for a fact that Belkar has only weeks left to live. Belkar on the other hand doesn't know this.
O-Chul also attempts to convince The Monster in the Darkness to turn against Redcloak and Xykon. He seems to have made quite a bit of progress by the time they part ways. Then again, the Monster is the Minion with an F in Evil.
Redcloak's backstory and continuous humiliation by Xykon have led to a lot of speculation that he's being set up for this.
His brother Right-Eye went through one in the backstory. He realized that working with a monster like Xykon would eventually lead to the destruction of the goblin people and that the grand Plan he and Redcloak had worked on their entire lives wasn't worth the sacrifice of so many goblins. This culminated in an attempt to kill Xykon for good; sadly, Right-Eye was Doomed by Canon.
Black Mage of Eight Bit Theater periodically comes close, but as soon as Fighter says something knife-worthy, he jumps right back to his Heel role, completely forgetting that he'd ever contemplated reforming.
Jacob Deegan has a pretty surprising Heel Face Turn in one of the most recent story arcs of the series, and actually manages a pretty heroic moment to boot.
Bulgak Adrak earns a particularly notable one, since it occurs while he is in Hell. A particularly explosive example, given the nature of souls in Dominic Deegan.
Vriska of Homestuck is a complicated example. She really, really wants to be a hero, but the problem is she has only half-formed ideas about how being a hero actually works, which means her actions tend to cause more harm than good. Ultimately, She is killed before she can become a true hero, although her appearances in the afterlife show she's now at least working towards a good goal, even if her methods are arguably genocidal.
Much later in Act 6, the Eridan half of Erisolsprite is genuinely remorseful for killing Feferi. Both halves of Fefetasprite forgive him.
Ixia at Cloudscratcher's beginning (though this was always her intention) and Sogaat prior to its beginning. Nyx might be headed this way.
The Inexplicable Adventures of Bob!: Both Galatea and the Treasure Monster started out basically as villains, though they both turned out to be very much of the villainous Woobie variety. Showing them some kindness and understanding pretty much did the trick. Bob tried the same approach with Riboflavin, who's much a more hardcore villain, and it didn't work.
Agents of the Realm: Jordan, who starts off working for The Dragon, joins the agents by the end of chapter four. Played with, however, in that while everyone knows by now that Ruby is evil, nobody knows what about Jade.