The Nega-Chin is the opposite of the Crimson Chin from a nega universe version of the Crimson Chin's world. He is a parody of Bizarro from Superman, although the latter means no harm unlike the Nega-Chin.
Nega-Chin is the Crimson Chin's evil doing twin from a parallel universe. Unlike most other villains that have appeared in The Fairly OddParents, the Nega-Chin is exceptionally capable, and has succeeded in tricking both the Crimson Chin and Timmy multiple times throughout the series.
The Nega-Chin looks almost exactly like the Crimson Chin, except with a dark gray suit and an anti-Chin symbol on his chin (the background's blue-violet and the c itself is blue and crossed out). His teeth are jagged and sharp. The spike on his head has points and he has red eyes in contrast to the Crimson Chin's yellow eyes.
The Nega-Chin was introduced in The Crimson Chin meets Mighty Mom and Dyno Dad and is known to be the Crimson Chin's evil twin from a parallel universe, he's just as strong, fast and good looking as the original. He switched places with the real Crimson Chin just before Timmy Turner wished to exit the comic book and back in his room. While trying to keep his parents busy, Timmy told Cosmo and Wanda to keep the Chin occupied with whatever he wants, including granting him wishes. Not knowing that they've actually granted an evil super villain ability to make wishes. In doing so, he makes it so Timmy can't wish him back to the comics, he brings the other super villains out from the comic so they can wreak havoc over Dimmsdale and to top it off, prevents Timmy from using his magic to clean up his room to which Timmy replies, "That fiend!".
While the Nega-Chin and the other super villains are destroying the town, Timmy wishes to bring back Mighty Mom and Dyno Dad (heroes) to take on the hoard of super villains as a great battle took place. However, the Nega-Chin got the upper hand as he used his Nega powers to strip Timmy's parents of their power, rendering them helpless. Just before he could finish them off, Timmy manages to save them by not just bringing out the real Crimson Chin, but also all of the other Crimson Chins from the past decades from the 30's to the new millennium Crimson Chin. With the force and might of the many Crimson Chin's and Timmy as Cleft the Boy Chin Wonder, they defeated the Nega-Chin as he escapes by wishing himself and his fellow super villains back into the comic book, swearing revenge on everyone (though the Crimison Chin claimed that he can't swear on it since only the 80s Chin can swear).
It wasn't until The Big Superhero Wish! that he planned his revenge from the last time they fought. He secretly sent Short-Fuse to the real world to let him know when the world became exactly like the comic book world so he can make a comeback and plan his revenge. When Timmy wished the world to be like a comic book full of superheroes due to his frustrations of real live heroes not helping him when needed, Short-Fuse quickly gets the news to the Nega-Chin, finding the wait surprisingly short. During Timmy's fight with his enemies taking the form of supervillains, disguised as the Crimson Chin, he sends the three to his lair and afterwards, he offers a deal to team up with them to take down Timmy as he knows the secret to his powers in the form of his fairies as Ace the Chin-Hound and Clefto.
Later, the trap was set as the super villains greatly over powered the kids as super heroes, including Cleft. Then Cleft tried wish away all the super heroes and villains, the Nega-Chin counters the wish by stopping Cleft after he said super heroes. Forced to be literal in wish granting, the disguised Cosmo and Wanda grants the wish, wishing away all super heroes, taking away their powers and leaving the Earth defenseless to super villains. Triumphant, the Nega-Chin mocks Timmy and kidnaps his fairies rendering them powerless. Later on, over confident on his victory, he didn't count on Timmy and the Crimson Chin in his alter ego, Charles Hampton Indigo, and the other kids and ordinary everyday heroes teaming up to take them on. Believing they'll win in a matter of seconds, while the other villains battle the remaining heroes, the Nega-Chin takes an evil shower. When done, he's surprised to see they actually defeated the other villains. Angered he attempts to blast them with his Nega-vision, but he accidentally breaks Ace and Clefto free from their prison. Timmy then quickly wishes the world back to normal, no one from his town would remember what happened and that the Nega-Chin be placed back in the comic. Bitter about his defeat, he wonders why he keeps losing despite all what he did, Timmy informs him that it's all the Crimson Chin Author's doing, since he does write the story. So the Nega-Chin plans to take down the author himself to finally win as he manages to escape. The episode ends with the Nega-Chin finding the author in the real world and beating him to a pulp. It is likely that this failed as well since he is not shown triumphing over the Crimison Chin.
The Nega-Chin was last seen as a cameo in the The Jerkinators! as he uses a newly found "Scepter of Nega-negatude", teaming up with the other Crimson Chin villains and defeating the Crimson Chin. But with the might of Cleft and Jimmy Neutron as "Brain Boy", he was easily defeated.