Reincarnated Individuals are people chosen by the goddess behind the Waves to act as her enforcers in the world(s) that she is fusing. These individuals appear to have been granted a special power by the goddess, such as Tact's ability to steal other people's powers or Ciel's ability to create a perfectly unbreachable barrier. They form an anti-thesis to the Heroes, as they directly work for the force that is trying to destroy the world and work to sabotage or kill the Heroes at whatever time they can. Since they can be recycled after death, Glass' world uses the Soul Eaters to completely destroy their souls after killing them and thus prevents their continued reincarnation.
It's been noted that Reincarnated Individuals have appeared throughout history. In Melromark's world they seem to have been used by the Goddess to destroy information that could aid the Heroes in stopping her plans.
The country of Faubley was completely taken over by one reincarnator named Tact, who then attempted to take over the entire world and use his power to steal the Legendary Weapons from their rightful wielders. Tact appeared to be under the impression he was actually saving the world, but because of his sheer selfishness, arrogance and stupidity was in fact leading directly to its impending destruction. He was defeated by the Heroes, Naofumi, Fohl, Trash, Ren and their comrades, and the weapons he'd stollen found new owners in Raphtalia, Firo, Windia, Wyndia and Midori. He was then promptly executed and his soul consumed by a soul eater. Trash and Naofumi then set about warning the countries of the world to keep an eye out for anyone who matches the description of a reincarnated individual and keep an eye on them to prevent another danger.
In Glass world the situation became much worse due to them not noticing the reincarnator in time. One reincarnator tricked several heroes into distrusting their comrades and killing each other. As many as two are known, one of them being Ciel, who believes the world is a game and that by killing his comrades and instead aiding in the Wave's advance he is letting the game install "Updates". Both reincarnators are noted to be in control of two large kingdoms. Eventually Ciel passes through the Wave and enters Melromark's world, calling it "A new world added by the Goddess' Update," and tries to force the Wave open to get in. He uses his barrier ability to hold off the heroes who desperately attempt to stop him from his goal. The heroes fail, causing the goddess, Medea, to descend upon the world. After Medea promptly kills Motoyasu, Naofumi and Ren, she sends her army of reincarnated to destroy what remains, allowing them to pillage and assault Melromarc for the sheer fun of it while all of them are still somehow under the impression that it's all a game, despite her openly admitting she intends to destroy the world, with them in it, when she's done.
The Reincarnated Individuals seem to be directly in contact with women like Malty Melromarc, who are in fact fragments of Medea, the goddess who reincarnated them in the first place.
Known Reincarnated Individuals
A Reincarnated Individual given the power to steal Legendary Weapons by the Goddess. He acts primarily on the belief that he's saving the world, when he's in fact aiding in its destruction. He is Lv.350 and has stollen five of the Vassel Weapons by the time he first appears but ultimately, since he's not actually a hero himself, his power is far inferior to the real Heroes once they get their bearings down and he is promptly defeated and fed to a Soul Eater.
A Reincarnated Individual from Glass world who is under the impression that he's playing a game and by letting the Waves destroy the world and fuse it with Melromark's world he is in fact letting it "Update" and add new lands for him to conquer and new equipment for him to gain. He has the ability to form a perfectly impregnable barrier for a limited time, but he and his allies cannot attack while under its protection, with the exception of the Scythe Vassel User.
Unnamed Scythe Vassel User
A reincarnated wielding the Vassel Scythe of Glass' world, like all of the others he believes himself to be playing a game. He is woefully weak normally, as without Medea's empowerment he was injured from a hit by Naofumi, the Shield Hero who has next to no attack power compared to the other heroes. He is also extremely arrogant, trying to claim he's smarter than Trash, despite only winning any of their battles by sheer force of numbers and relying on Medea's power whenever they were going to lose. He apparently has a harem and tried to add Melty to it after defeating her army, regardless of her will however he decided to kill her instead when she attacked back. Naofumi fortunately prevented this from happening. He appears to be able to attack through Ciel's barrier, meaning he can attack while Ciel defends.
Unnamed Blade Vassel User
A reincarnated wielding the Vassel Blade from Glass' world, like all the others he believes himself to be playing a game. He is naturally much weaker than Melromarc's heroes, much like all of his fellow reincarnated, and he would easily be defeated if not for Medea's empowerment.