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. They had been reincarnated by malign deity. They seem to have been used by that deity to destroy information that could aid the Heroes.

Personality
All of the reincarnated individual have a common characters. They are all prideful, arrogant and self-proclaimed geniuses. All of them think of this world to be games and doesn't bother about the life inhabiting the world. They don't really care for the wave and are trying to fulfill their own selfish delusional desires. They mostly refused to listen to others opinion or reasons and only tries for their own satisfaction, Such as, forming a harem, being king, and calling themselves the heroes.

They are overconfident and try to stand out as much as possible while some choose to hide their power. As Naofumi stated, most of them are otaku or gamer in their original world and try to out shine in this world. They believe in their self-righteousness, as they are chosen by a being called god.

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. They cause chaos, confusion and mess things up with their twisted or other worldly logic. In times of their demise, all of them proclaims that they will return or again reincarnate or demand a redo from someone unknown. They are mostly responsible for the loss of many important information and also loss of order in the world.

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. Butm Itsuki claims that these individual may be same as him with Esper ability and that the Goddess 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. In the LN, Raphtalia uses the Glass world's vassal weapon Katana to devour the soul of reincarnated. As she did with Kyo's and other two reincarnated souls.

It is interesting to note that the vassal weapon of Katana choose Raphtalia over the reincarnated Trash #2.

History
Elrasla Grilaroc of Hengen Musou stated that "Geniuses 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 so he 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 Naofumi's comrades). In the WN, after facing Tact, Naofumi and other heroes come to the conclusion that they were not geniuses but was actually reincarnating people from another world with their knowledge intact from their world.

After his weapons was stolen away, Naofumi 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, Nafoumi  pieces together the information of the Queen, granny and spirits and concluded that these Reincarnated one are chosen by the one causing this Waves to act as an enforcers in the world(s) that is involved in fusing.

Like Hengen Musou, many techniques, technology and other means that gives advantage to fight waves used by the past heroes disappeared  in passage of time. Naofumi predicts 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 if these individual acted on their own ideas & belief, they succeeded in fulfilling the what the gods wanted.

Naofumi finds that they succeeded in fulfilling their role as, the way to fight Houou and the Spirit Turtle, the means to break the Level Cap of 100 & levels beyond, forgotten way for mass-producing the replica of holy weapons, the destruction of the Hengen Musou School and various pieces of information were conveniently eaten away by the ages. As these information of great values, cannot disappear in natural way. The uselessness of the information Ren, Motoyasu, and Itsuki brought to this world all supported his theory. After being able to awake the brainwashing power of the bow by feeding unknown material to his bow, Itsuki himself also felt that his knowledge of the game with convenient information regarding the waves were erased from his memory.

Naofumi also realized that the weapon strengthening 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.

In Glass world was also left with information on what the wave was, but everything else was lost over time. Naofumi find this ancient information on Glass world regarding killing the other world holy heroes to prolong their world to be nonsense and suspect it to be fabricated, as one of the holy weapon (Hunting Hero) in her world lacked fighting ability much like himself, if their concept were true, all the heroes need to excel in combat .And also because their world vassal weapons assisted him & his party to take down Kyo  One vassal weapon, Katana, even choose Raphtalia as its wielder and went to Raphtalia's own world. which all made Naofumi doubt the ancient theories of Glass's world

Naofumi begins to suspect that the one who claims to be the Hero in the past who wielded the brainwashing dagger could also be a reincarnator, to undermine the hero's faith among the people. As Itsuki's brainwashing curse series was achieved forcefully by feeding an unknown material to the bow by the rebellion faction support this theory. Rishia & Melty also claims that though there was a brainwashing weapon in the past, there was no evidence that it was the shield hero, humans arbitrarily believed it to be the shield hero for their resentment towards him for favoring the Demi-humans.

Trash also confirms that there were religion that worshiped a being God other than the hero faith and suddenly it lost it's traction, and people begin to shift to hero faith with strong devotion, the reason for which remained unknown. It could be that the people of the past came to realize something and realized that the heroes are the one could save them from the calamity.

Summary
It's been noted that there were rare geniuses who have appeared throughout history in both Melromark's world and also Glass's World. Naofumi finds these geniuses to be just a Reincarnated Individuals retaining their previous world's knowledge

Naofumi names the one who mediates reincarnations individual as God, based on the novels he read in his world. He clarifies that it claims to be god and not really divine. According to the novel Naofumi read, If someone were to die in a tragic accident, he would call out to them. To those who faced such misfortune, he would say that you died an unfortunate death, I will give you life once more in another world?. It is nothing but a tempt. Since they're dead, there's no way they can refuse. Even If they decline, god will tempt by words of developed a liking for them, so give them a cheat. in away forcing them to agree either way. There's also the possibility of forceful reincarnation without any interaction. Knowing that  God was the one who reincarnated them, they feels special and develop a superiority complex and doing things of their own self-righteousness. If the reincarnated one was a gamer than he sees this as nothing but a game. They are just a playing piece that cuts off the lineage that aids the heroes.

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 stolen 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. Their shout of wanting to stop the wave and their reason of it being the world fusing together resulting in not even a blade of grass to grow feel on their deaf ears. 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. Raph-chan pointed out his soul which appeared to be that of the middle aged man, but Nafoumi ordered Rishia to throw the ofuda and fed his soul to the Soul Eaters, but he vowed to be back.

Unnamed Reincarnators
According to S'yne two of them destroyed her world by killing the holy weapon wielders. Now they were aiming to kill Naofumi and other holy weapon wielders of the world. When Naofumi tried to reason with them, they just that they wanted to prove that their world is the strongest and also they kill to get bonus & powers for themselves. Disgusted with them, Nafoumi and his team killed them but they kept on reappearing. Finally, after them being killed for the third time, Raph-chan pointed out their soul which were destroyed by Raphtalia punting an end to them

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. Before his execution he demanded a redo to an unknown entity, so he would again reborn in this world to avenge himself, But the heroes just fed him to a Soul Eater to prevent such scenario.

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.

Controlled ones
After Medea descend to the world she reincarnated many to fight the heroes side, in order to amuse herself by seeing their distress in fighting their loved and trusted ones.

Mirellia Q Melromarc
In order to amuse herself, Medea reincarnated late Mirellia to lead Medea's reincarnated force against the heroes, much to the dismay of Trash who couldn't bear to see it. Medea herself revealed that Mirellia was not acting off her own free will but controlled by Medea. Eventually, Trash found away to free her soul from the clutches of Medea and she decided to fight Medea along with her family and Heroes, but Trash asked her to leave everything to him, as by exerting every magic may lead her to nothing

Elrasla Grilaroc
She is one of the few to be reincarnated ones to trick the Heroes side and pretend to fight for them but turn against them when the Medea's force have an advantage. She tried to attack trash and Filo when the time was right but was exposed. In final battle, She was confronted by both Rishia and Eclairs, somehow successfully brought to her senses and she fought along with the heroes against Medea.

Other Rock valley villagers
Many rock valley villagers who were killed were reincarnated and send to the heroes in how of creating a confusion and strike the hero side when they are off guard. Luckily Sadina was able to see through them being controlled and warned the others. Some were even killed by Sadina herself.