Kaiju No. 8 is an exciting tale following humanity’s struggles against monsters known as Kaiju. The series has introduced several of these intimidating creatures with unique powers and personalities, but the primary antagonist appeared only in the recent chapters of the manga.
As a result, those who are not caught up with the story may be curious about this enigmatic figure. The main villain of Kaiju No. 8 is the Meireki Era Mega Monster, also known as the Daikaiju of the Meireki Era, or Meireki. This creature has only shown up in the current Last Wave Arc of the manga, but has already caused rampant destruction.
Most fans of Kaiju No. 8 who are yet to read the newer chapters of the manga may mistakenly believe that the main antagonist of the series is Kaiju No. 9. This Kaiju has certainly played the biggest role in the plot so far, absorbing Isao Shinomiya and leading large-scale attacks against the Defense Force. However, after the cast worked together to defeat Kaiju No. 9, the greater power behind it was revealed - the Meireki Era Mega Monster.

The Meireki Era Mega Monster is a legendary Kaiju that has existed for over 400 years. It had been fused inside Kaiju No. 9’s body and only appeared after its defeat. Unlike Kaiju No. 9, who is endlessly curious, Meireki is a purely devastating force that cares about little else than wreaking havoc.
Meireki is considered the ultimate beast by the Defense Force from the moment of its arrival. Its massive stature, multiple limbs, and several heads are enough to instill fear in the strongest of soldiers. Having existed for centuries, Meireki possesses unsurpassed knowledge and power.
It is fused with several other Kaiju, allowing it to use their special powers. As the guiding force behind Kaiju No. 9, Meireki possesses all its intellect and can also control the Kaiju created by the former antagonist. It can release powerful energy blasts, use necromancy to manipulate Kaiju's dead bodies, and place lethal runes on targets.
Additionally, it can also use various forms based on the bodies it has absorbed. Hence, Meireki has shown a more compact figure for close-combat confrontations, but transforms into a large beast when the tides of battle turn.
Right after appearing, Meireki immediately fatally harmed Kafka and plunged the entire Defense Force into despair. Despite Narumi and Hoshina’s combined efforts to face it, they were also overpowered. Right when humanity seemed to be on its last legs, Kafka regenerated with his new core. He then unveiled a new form that drastically enhanced his powers, after which he engaged in an electrifying fight against Meireki.
The recent manga chapters saw Mina and Kikoru working together to harm Meireki, forcing it to reveal its shielded core. Kafka was preparing a final attack to destroy Meireki once and for all. However, since this creature is the main villain, it is unlikely that it will die so soon.
Did the Meireki Era Mega Monster create Kaiju No. 8?
Kafka was turned into Kaiju No. 8 after a mysterious larva entered his body. When the protagonist was seemingly killed by Meireki, he experienced the reason behind his transformation into a humanoid monster. This revealed a 400-year-old grudge against the Meireki Era Mega Monster.
In 1657, Kaiju slayers, with their parasitic weapons, were fighting an unknown monster for the sake of protecting their homeland. Unfortunately, they ended up being wiped out, as their opponent, the Daikaiju of the Meireki Era, was too destructive.
Meireki continued its slaughter for 100 days, burning down everything the Kaiju slayers held dear. Feeling the weight of their powerlessness, the resentments of all the soldiers gathered to create a larva. This larva was the core of Kaiju No. 8, which finally found its suitable host in Kafka.

Once the protagonist got to know the secret behind his power, he sympathized with the struggles faced by the larva. With the opportunity to return to life as a normal human or create a new core with his heart, Kafka decided to remain a monster in order to honor his predecessors.
Therefore, as mentioned, he returned to life and fought against Meireki in his new form, driven by the regrets and unfulfilled wishes of his ancestors.