Vanguards of the Waves

Reincarnated Individuals are people who were reborn into the world of heroes and they retain their memories of their previous life.

Personality
All of the reincarnated individual are prideful, arrogant and self proclaimed geniuse. They don't really care for the wave and are trying to fulfill their own selfish deluding desires.They are overconfident. Such as, forming Harem, Being King  and calling themself the heroes.They mostly refused to listen to the opinion or words of others and live their own life.All of them think of this world to be games and doesn't bother about the life inhibitat there and try to stand out as much as possible while some choose to hide their power.As Nafoumi stated, most of them are loser in their orginal world and try to out shine in this world.

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.

History
Granny of Hengen Musou stated that  Geniuse were the apostles of social growth and decay. One such individual was under the impression that he was the one worthy of ruling the world, and Musou teachings forbid such ways of thought and ended Hengen Musou teaching by starting an internal discord.This eventually led to the downfall of the technique and lost in time, except for Granny ( who later pass it on to Nafoumi's comrades).In WN,  After facing  Tact, Nafoumi and other heroes come to the conclusion that they were not geniuse but was actually reincarnating people from another world.

After his weapons was stolen aways, Nafoumi was in the Spirit world were the spirit reveal that the one who cause this interference or wave kept on sending enemies(reincarnated individual), no matter how many decades, or centuries. After listening to the story and piece together the information of the Queen, granny and spirits.Nafoumi concluded that these Reincarnated one are chosen by the self proclaimed god(dess)  the one causing this Waves to act as her enforcers in the world(s) that she is fusing.

Like Hengen Musou, many techniques, techonology and other means that gives advantage to fight waves used by the past heroes disappeared  in passage of time. Nafoumi state that the one behind it used this reincarnating people from another world and control them. To take away the means to fight against the Wave.and to make sure these powers would fade away one day.

Even though these individual acted on their own ideas & belief, they succeded in fulfilling the what the gods wanted.

Nafoumi finds that they succceded in fulfilling their role as, the way to fight Houou and the Spirit Turtle, the means to break the Level Cap of 100 & levels beyonds, the destruction of the Hengen Musou School and various pieces of information were conveniently eaten away by the ages.The uselessness of the information Ren, Motoyasu, and Itsuki brought to this world all supported his theory. Itsuki himself also felt that his knowledge of the game with convienint information regarding the waves were erased from his memory.

Nafoumi also realised that the weapon strengtening method of the past heroes were also destroyed and suspect that important information couldn't be forgotten but deliberately destroyed by someone. Fitoria also told that her master asked her to stop the heroes from fighting each other, as  many heroes in the past fought each other instead of joining  to fight against the waves, Nafoumi suspects that some reincarnated individual instigated these fights.

Abilities
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.But Itsuki claims that these individual may be same as him with Esper ability and that the Godess choose the one with suitable Esper for her work.

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.

Summary
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.

In Glass' world they have had an even more adverse influence but rather in the opposite direction, they have instead increased knowledge on the Heroes' weapons and mission but warped it to serve their goals. Two such individuals are known, Trash #2, as Naofumi calls him, and Kyo. The former formed a harem, introduced new technology and wanted to become the Vassel Katana's master, however he failed and the weapon chose Raphtalia instead. The latter was a significantly greater threat as he worsened the Spirit Turtle rampage, and then stole the Holy Beast's power when he was defeated thus having a massive negative impact on Melromark's world before he returned to his own. This sin was bad enough that his own Vassel Weapon, The Book essentially called out to Glass' group to execute him and when even that wasn't enough Naofumi, who cannot normally cross worlds as he is a Holy Hero, was granted an exception in order to exterminate him.

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.

Trash#2
A Reincarnated Individual from the Light Novels. He had a sizeable harem and was an inventor who created clones of the Sacred Beast, the White Tiger, and also found ways to prevent the Heroes from warping away to safety. He wanted to get the Vassel Katana but became envious and wrathful when Raphtalia got it instead. He is backed by his nation in hunting Raphtalia down but he ultimately gets himself killed when Raphtalia subdues him in a way that if he moved before receiving healing magic he would die, this tactic was only meant to allow Raphtalia's group to escape but Trash#2 ignored the warning and thus got himself killed.

Kyo
A reincarnated individual from the Light Novels. Like most of them he is a "genius inventor", while he is evidently quite smart and competent at strategy he is insufferably smug about it to the point he thinks he's much more so than he really is. He caused and worsened the Spirit Turtle rampage of the Light Novels and he also stole some of its power when he was defeated, power fueled from every living being it killed under his control. He fled back to Glass' world before ultimately being tracked down and defeated there.

Tact Arusahorun Faubley
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 stolen 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.

Ciel
A Web Novel only Reincarnated Individual from Glass world and the only one named amongst them. He 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 Web Novel only 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 Katana Vassel User
A Web Novel only reincarnated individual wielding the Vassel Katana 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. Given his connection to the Katana he may be Trash#2 or the prototype for that character.