Shinkaiou

Shinkaiou is the king of the Sea People (called the Deep Sea Tribe in the anime adaptation), a race of aquatic humanoids regarded by Genus-hakase as a naturally occurring species and a kind of “natural enemy” of humanity rather than artificial monsters.

He suddenly launches an invasion on J-City, claiming that the true rulers of Earth are the denizens of the sea and that humans are merely squatters to be wiped out.

His Disaster Level is officially classified as Demon.

In the broader narrative, he serves as the clearest early example of how terrifying a Demon-level threat really is, contrasting sharply with earlier “Demon” monsters whom Saitama dispatched effortlessly.

According to the official guidebook “Hero Encyclopedia,” Shinkaiou stands about 4 meters tall when dried out on land, and when he rehydrates in the rain and returns to his “true form” as a sea creature, he grows to around 20 meters.

Despite his almost comical design elements (such as the heart-shaped nipples and exaggerated bodybuilder frame), his portrayal in both manga and anime emphasizes sheer menace and brutality.

His speech is characterized by a “camp” or “okama-style” tone in the original, which comes across as flamboyant and contemptuous, especially in lines like “You’re unpleasant, so I don’t mind if you die.”

The anime voice actor for Shinkaiou is Rikiya Koyama, whose performance heightens both the theatricality and horror of the character.

Invasion of J-City

Shinkaiou appears leading an organized assault on the surface, with numerous Sea People attacking various scenic coastal tourist spots and urban areas.

They arrive riding a massive tsunami, though the exact mechanism by which this wave is generated is never explained.

When several of his subordinates are killed by heroes, Shinkaiou personally comes ashore to retaliate in J-City.

At this stage he is not yet fully hydrated and is still in his smaller, 4-meter “dried-out” state, but he is already more than enough to overwhelm most professional heroes.

Battle Against A-Class and S-Class Heroes

Shinkaiou quickly crushes two A-Class heroes, Stinger and Inazumax, effectively one-sidedly disabling both of them to avenge his fallen kin.

Their defeat underscores the enormous gap between mid-rank heroes and a serious Demon-level threat.

Soon after, the S-Class hero Puri-Puri Prisoner arrives.

Shinkaiou and Puri-Puri Prisoner engage in a direct power slugfest; despite the S-Class hero’s transformation and full power strikes, Shinkaiou dominates the exchange, pummeling him with a barrage of punches described as having “murderous intent in every blow,” and ultimately defeating him.

The wanted S-Class-level fighter Onsoku no Sonic is also present.

Initially, Sonic’s speed allows him to dance around Shinkaiou’s attacks and appear to gain the upper hand, as the sea king struggles to track him.

However, the weather shifts and rain begins to pour.

As Shinkaiou absorbs water, he regenerates damage, rapidly recovers his strength, and “returns to his true form” as a gigantic, over-20-meter-tall sea monster.

In this fully hydrated state, his power and speed increase dramatically, to the point he can now keep up with Sonic’s movements and even outmuscle the S-Class heroes he had already defeated.

Sonic, realizing the huge jump in threat level, is forced to retreat instead of continuing the fight.

Attack on the Evacuation Shelter

Having brushed aside the heroes, Shinkaiou heads to an emergency evacuation shelter where J-City civilians have taken refuge.

The shelter is built strong enough to withstand missile strikes, but Shinkaiou pierces it with raw physical force, demonstrating how absurdly powerful his blows are.

Inside, a group of relatively low-ranking heroes—Sneck, Jet Nice Guy, All Back-Man, and Bunbunman—decide to stand their ground to protect the civilians.

Despite their courage, they are instantly overpowered and beaten nearly senseless.

At this critical moment, Genos arrives to fight Shinkaiou.

Genos initially appears to have the advantage, blasting the sea king with powerful attacks and even sending him flying once, but he fatally underestimates Shinkaiou’s durability and regenerative potential.

The Genos and Mumen Rider Moments

During their battle, a young girl among the evacuees cheers for Genos, which annoys Shinkaiou.

In response, he spits a stream of corrosive fluid directly at her, intending to kill her purely out of spite.

Genos moves to shield the girl and is struck by the acid, whose destructive power is so high that even his fully mechanical body is rendered nearly inoperable in a single hit.

Shinkaiou then tears off one of Genos’s arms, exploiting Genos’s brief moment of carelessness.

Despite losing an arm, Genos continues fighting and manages to trade blows with Shinkaiou, indicating that in raw stats he is not outclassed.

However, protecting civilians and being caught off guard by the acid are decisive factors; in the end, Genos collapses from damage and Shinkaiou prepares to finish him.

Before the killing blow can land, Mumen Rider, a C-Class hero who had been pedaling through the pouring rain, arrives and challenges Shinkaiou.

Mumen Rider knows he is hopelessly outmatched but fights anyway, becoming a symbol of heroic resolve for the watching civilians.

Shinkaiou brutally beats Mumen Rider, knocking him down again and again, but Rider keeps standing back up, empowered by the cheers and support of the shelter’s survivors.

Ultimately, Shinkaiou crushes him and sends him flying, leaving him broken and unconscious.

Defeat by Saitama

Immediately after Mumen Rider is defeated, Saitama arrives on the scene.

Shinkaiou, brimming with confidence after having routed two S-Class heroes and several others, confronts Saitama head-on.

Saitama responds with a single punch.

This one blow creates a massive hole in Shinkaiou’s torso, killing him instantly and completely ending the sea people’s invasion and their plan for surface domination.

Shinkaiou’s death, following such an intense build-up of fear and despair, serves as a classic One-Punch Man contrast: the overwhelming threat of a major villain versus Saitama’s casual, effortless power.

It also dramatically reinforces how far above even top-tier Demon threats Saitama truly is.

Disaster Level and General Threat

Shinkaiou is officially rated as Disaster Level Demon.

He is repeatedly referenced in later materials, such as the Volume 15 bonus chapter “Disaster Levels” and Genos’s comments during the Monster Association raid, as a “textbook” example of a Demon-level threat.

Even in his somewhat weakened, dried-out land form, he is capable of feats like collapsing high-rise buildings with his bare hands.

He can also smash through an evacuation shelter designed to endure missile impacts, purely with physical strikes.

Narratively, he demonstrates that for the vast majority of heroes—especially A-Class and below—a Demon-level opponent is practically insurmountable.

Even S-Class heroes can lose or be pushed to retreat if the matchup and circumstances are unfavorable.

Physical Strength and Speed

Shinkaiou possesses tremendous physical strength, even before rehydration.

He easily dominates Puri-Puri Prisoner in a contest of raw power, pummeling him with a series of lethal punches.

His speed is also noteworthy: he can move fast enough to appear behind a distant opponent like Inazumax before the target even senses his approach.

This level of agility, combined with his bulk, makes him extremely dangerous in close combat.

After soaking in the rain and returning to his “true” sea-creature form, his physical power and speed increase dramatically.

In this massive 20-meter form, he can match or surpass the speed of Onsoku no Sonic in bursts and ragdoll S-Class-level opponents like Genos.

Special Techniques and Biology

Shinkaiou’s arsenal includes more than just punches and kicks.

He has several distinct biological weapons and traits:

“Internal Moray” Tongue Weapon

He can extend a moray eel-like organ from his mouth, equipped with sharp fangs and able to stretch and retract like a tongue.

This weapon is used as a sudden, unpredictable strike that can pierce and tear opponents.

Corrosive Fluid (Acid)

Shinkaiou can spit a highly potent corrosive liquid from his mouth.

Direct contact with this acid is powerful enough to render the fully mechanical Genos nearly combat-ineffective in a single hit, showcasing devastating offensive potential.

Regeneration and Toughness

His body is extremely durable, such that attacks from S-Class heroes often fail to inflict critical damage.

Furthermore, as long as he can absorb water—especially rain—he can regenerate minor to moderate injuries quickly.

Hydration-Dependent Power-Up

Being an aquatic organism, Shinkaiou suffers some power loss on dry land as his body dehydrates.

When it rains, or when he otherwise absorbs sufficient moisture, his strength, speed, and size all scale upward drastically, turning him into a kaiju-like giant.

Power Level Discussions and Comparisons

Some fans have questioned whether Shinkaiou in his fully hydrated form might qualify as Disaster Level Dragon, given that he disabled or forced the retreat of S-Class heroes like Puri-Puri Prisoner and Genos and kept pace with Sonic.

However, all official references to date—Volume 15’s “Disaster Levels” chapter, drama CD “Maji CD Vol. 02,” and other materials—consistently classify him as Demon, not Dragon.

In drama CD “Maji CD Vol. 02,” a simulation shows that even a past, weaker version of Genos (from the era when he could not defeat Asura Kabuto at all) is able to defeat Shinkaiou in training scenarios.

This reinforces the idea that while Shinkaiou is terrifying to most heroes, he is below the true Dragon-tier heavyweights.

The story also notes that heroes like Stinger, Sneck, and Mumen Rider all survive their encounters with Shinkaiou despite being decisively beaten.

This is not due to Shinkaiou holding back—he is shown to have genuine intent to kill—but rather highlights how physically tough even low- to mid-tier professional heroes are compared to ordinary humans.

Matchups and Author Commentary

In an old blog Q&A, when asked if Onsoku no Sonic, properly equipped, could defeat Shinkaiou, ONE responded that killing Shinkaiou outright with bombs or blades would be difficult due to his durability and regeneration.

At the same time, he suggested that Shinkaiou might never land a hit on Sonic if Sonic’s speed and tactics were optimal.

In a live Nico Nico broadcast interview, ONE was asked whether anyone other than Genos could defeat Shinkaiou.

He answered that it would be tough without high offensive power, mentioning that someone like Zombieman would struggle, while stating that the current Sonic, with weapons and in non-rainy conditions, could defeat Shinkaiou.

This places Shinkaiou as a high-end Demon-level opponent who is particularly dangerous in bad weather or aquatic environments, but not an insurmountable wall for elite, specialized fighters.

He is also strongly implied to be much more threatening in the ocean itself, though we only ever see him fight on land, which he calls an “away game” for him.

Sea People as a Whole

The Sea People are a race of beast-like aquatic humanoids that attack the surface alongside Shinkaiou.

Many of the named or focused Sea People are classified as Disaster Level Tiger, marking them as serious but not civilization-level threats when acting individually.

Compared to Shinkaiou’s more humanoid, “ruler-like” appearance, many Sea People soldiers are even more bestial, resembling various sea animals fused with humanoid forms.

In the anime, they are collectively called the “Deep Sea Tribe” to match voice line rhythm and naming.

Their invasion is notable for coordinated assaults on coastal tourist attractions and:

The use of a massive tsunami for surprise attack and rapid deployment.

The implication that Sea People infrastructures or powers may allow them to manipulate the sea, though the story does not spell out the exact method.

Despite their numbers, once Shinkaiou is killed by Saitama the invasion effectively collapses.

Almost all Sea People are wiped out during the conflict, leaving very few survivors.

Hohoshiro (Second Deep Sea King Candidate)

In a manga side story titled “Catch & Smash,” a Sea Person named Hohoshiro appears.

He resembles a great white shark in humanoid form and is presented as the last surviving Sea Person after the failed invasion.

Weakened from earlier battles with heroes, he hides in a pond within a wide-area nature park in K-City to recover.

After consuming enough water and nutrients, he reemerges, declaring himself the “Second Deep Sea King” and vowing to stand at the top of the food chain as the final survivor of his race.

Unfortunately for him, Saitama happens to be in the park after hearing rumors about a monster sighting.

When Hohoshiro eats the pond’s resident creature—the “master of the pond”—Saitama becomes annoyed and punches Hohoshiro once.

Just like Shinkaiou, Hohoshiro is killed instantly in one blow.

This brief episode underlines both how casually Saitama ends major threats and how completely the Sea People’s ambitions have been extinguished.

Shinkaiou is noted to have been on roughly equal footing with Chitei Ou (the subterranean king) and Sky King, forming a trio of rival “kings” representing sea, underground, and sky.

After Shinkaiou and Chitei Ou die, Sky King attempts a surface invasion of his own.

Sky King is quickly obliterated by Melzargard, a Disaster Level Dragon alien monster who treats him like an insignificant insect.

Chitei Ou, for his part, had previously been killed by a single punch from Saitama, much like Shinkaiou.

From Shinkaiou’s feats, it is inferred that these other “kings” were likely also powerful Demon-level threats.

Under certain conditions, they might be able to give low-level S-Class heroes serious trouble, and A-Class heroes would generally stand little chance.

Later commentary and side stories emphasize that Shinkaiou is one of illustrator Yusuke Murata’s favorite monsters, along with Boros, leading to especially elaborate and imposing designs in the manga.

Murata’s reimagining of ONE’s simple original designs into more detailed, intimidating forms increases the sense of dread that characters like Shinkaiou are meant to invoke.

Shinkaiou combines regal arrogance with sadistic cruelty.

He has zero respect for humans, viewing them as pests infesting his planet, and freely threatens and kills civilians without hesitation.

He is easily irritated by displays of hope or defiance, such as a child cheering for Genos.

His line to that child—basically stating that she is unpleasant and therefore “has his permission to die”—captures his casual cruelty.

At the same time, he is not purely bestial or mindless; he talks, reasons, and reacts to opponents’ actions with a certain theatrical flair.

His campy, effeminate speech style contrasts sharply with the brutality of his actions, making him simultaneously unsettling and darkly humorous.

He also shows some capacity for surprise, such as when Genos sacrifices his own body to protect a child.

However, these moments never translate into mercy; they only slightly amuse or bemuse him before he resumes his attempt to kill.

In interviews and commentary, Shinkaiou is cited as a favorite monster of artist Yusuke Murata.

This affection is apparent in the highly detailed and dramatic panels of Shinkaiou’s battles, particularly his transformation under the rain and his clashes with Genos and Mumen Rider.

Murata’s design keeps the essence of ONE’s original concept—an overconfident, flamboyant sea king—but greatly amps up the visual impact.

This results in scenes that leave readers with a powerful sense of despair before Saitama arrives to flip the tone completely.

The existence of the Sea People, and of Shinkaiou in particular, reinforces the setting idea that “monsters” are not always artificial or magical.

Some are simply apex predators or evolutionary branches that stand as natural enemies to humanity.

In the anime’s next-episode preview for episode 10, Shinkaiou himself narrates that the only “miscalculation” was that the fight took place on land rather than in the sea, calling it an “away game” for him and claiming he would be even more formidable underwater.

Still, considering Saitama’s performance against every other opponent, it is heavily implied that even in his preferred element, the outcome against Saitama would not change.