Just as seawater and Seastone are the natural enemies of all Devil Fruit users, Haki also has a strong suppressive effect on Devil Fruit abilities. That said, this suppression is nowhere near as absolute or brutal as seawater or Seastone.
How effective Haki is against a Devil Fruit user depends entirely on the user's level of Haki mastery.
Take the Atsu Atsu no Mi user in front of them as an example. If what he claimed were true, and his body temperature could really rise to ten thousand degrees, then with Yemu's current level of Armament Haki, there would be no way to defend against that heat at all.
But that was clearly impossible. If such a temperature were truly reached, the entire Scrap Island would have vaporized in an instant.
Even so, judging from the fact that this Atsu Atsu no Mi user could confidently melt steel and remain unfazed by magma, his temperature was likely at least two to three thousand degrees. That was roughly the level Yemu's most reliable technique, the Inferno Prison Dragon Fist, could barely achieve.
And yet, for this Atsu Atsu no Mi user, reaching that level seemed effortless.
How could Yemu not covet such an ability?
Relying purely on hard training was certainly fine, but when it came to protecting himself, having a few extra trump cards was never a bad thing.
...
"Shinto Sword Style: Spiral Thrust..."
As Erik's voice fell, he spread his arms, holding a blade in each hand at his sides. With a powerful push from his rear leg, his entire body surged forward, spinning as he thrust.
A tornado formed visibly from the high-speed rotation, charging straight toward Aquino.
Seeing this, Aquino grinned. The heat radiating from his body surged upward as white steam erupted around him, spinning rapidly. He then pulled his hands apart.
"Atsu Gesho!!!"
The white steam tornado moved as if under his command, slamming straight into Erik's Spiral Thrust.
The whirlwind created by Erik's rapidly spinning twin blades collided head-on with the scorching steam tornado.
"Oh… you can actually block my Scorching Infusion?"
Seeing the two forces locked evenly against each other, Aquino looked slightly surprised, though only for a moment.
"In that case, try this! Heat output increased to 5,000 degrees, Atsuyaki Tamago!!!"
One after another, fiery red spherical gas bombs filled with searing heat shot toward Erik.
"What terrifying heat..."
Feeling the intense temperature radiating from the spheres, Erik was genuinely taken aback.
"But this speed… it's way too slow."
"Shinto Sword Style: Wind Net."
Erik's twin blades flashed as he swung them rapidly. A clearly visible net formed from compressed wind blades, appearing directly in front of the incoming Scorching Explosive Bombs.
Boom… boom… boom…
A series of violent explosions rang out as thick white smoke engulfed both fighters.
...
On the deck of the Twin Goddesses.
Watching the battle unfold, Flare recalled a scene she had once witnessed in Loguetown and asked Yemu with a puzzled expression.
"Why is it that this guy's Devil Fruit clearly looks powerful, and the temperature is so high, yet the actual damage doesn't seem that impressive...?"
"It's simple," Yemu replied calmly. "Tell me, which hurts more, being hit by air, by water, or by a rock?"
"Of course it's the rock, Yemu-sama!"
Baby-5 answered without hesitation.
Yemu smiled and gently patted Baby-5 on the head in encouragement before continuing.
"That's exactly it. This guy's Atsu Atsu no Mi really is powerful, but all he can do is raise the heat. He can't actually control it. So unless he gets in close, the only way he can attack is by heating up the air and moisture around him. My Mera Mera no Mi, on the other hand, produces real flames. It's like water flowing. Sure, it doesn't hit as hard as a rock, and the temperature can't compare to the Atsu Atsu no Mi, but because I can shape and control the flames, I can still boost the power of my Mera Mera no Mi."
"And the Magu Magu no Mi is the rock. Maybe its temperature is a bit lower, and its transformations aren't as flexible, but rock is rock. No matter what, it's always going to hit a lot harder than water."
"On top of that, this guy's stamina and martial arts are terrible. It's honestly a waste of a Devil Fruit like that..."
As if to prove Yemu's point, after a prolonged exchange, Achino still couldn't do anything to Erik. Even with such a strong Devil Fruit, he was completely helpless.
"Damn it... huff... huff..."
Achino was panting now, and that actually surprised Erik.
"Is that all you've got? With power like this, you really thought you could come after the Sky Pirates...?"
"What?!"
Those words instantly set Achino off.
"What do you mean, 'all I've got'?!"
Erik, as if he couldn't even hear the anger in Achino's voice, answered with a straight face.
"You're too weak. Other than your Devil Fruit, there's nothing about you that stands out."
Achino's whole body shook with rage.
"You're saying you can beat me?! You can't even get close to me, and you still dare to talk like that?! Fine. Then I'll let you see my strongest power!"
Without hesitating for even a second, Achino's body temperature began skyrocketing.
Erik hadn't even had time to react when the two sons and one daughter hiding behind Achino were already terrified.
"No, Dad! You'll destroy the whole island!"
Achino didn't care in the slightest.
"It'll be fine. Pfft—"
A huge amount of steam burst from all over Achino's body, and his entire form slowly lifted into the air.
"Your dad can fly. Heat rising to 10,000 degrees."
The moment he said it, Achino's two sons and daughter didn't hesitate at all. They turned and sprinted toward Water 7.
Meanwhile, Achino's body grew brighter and brighter to the naked eye, dazzling and hard to look at.
The temperature of the air over the entire island surged rapidly. Even ordinary people far away in Water 7 could feel the heat changing.
Scrap Island was made mostly of wood, so there was no way it could withstand a 10,000-degree inferno.
Starting from Achino's body like the center of a spiderweb, the ground began to burn at frightening speed, turning to charcoal, then vaporizing as the fire spread outward and ash rolled across the island.
Erik watched the scene, stunned.
"What a terrifying ability… no wonder the captain had his eye on it. But this is Mr. Tom's island. I can't let you destroy it like this..."
"In that case, I'll end this with my strongest strike."
As he spoke, Erik raised his twin blades high and crossed them. A strange current began to gather around the blades, clearly visible as it swirled.
Not far away, watching closely, Yemu asked in curiosity.
"What is that? It's not wind… and it's not Haki..."
The one who answered was Erik's sister, Ayala.
"That's the ultimate secret of the Shinto Sword Style my brother practices. Only someone who has stepped into the realm of a sword master can wield it: the Spirit of the Divine Path."
"The Spirit of the Divine Path?!"
"Yes!"
The moment Ayala finished speaking, Erik's slash was unleashed.
"Shinto Sword Style · Ultimate Technique · Spirit of the Divine Path · Five-Colored Peacock!"
In the sea of blazing red, the instant Erik brought his blades down, a green slash tore forward.
No… not one.
Five.
The green slash split apart mid-flight, one dividing into five separate blades.
This time, it wasn't just Achino who froze. Even Ayala on the ship was stunned.