This tied into some discoveries Yemu made while developing his Devil Fruit abilities.
Though Yemu wasn't a scientist like Vegapunk, as a Devil Fruit user, someone who had gone through nine years of compulsory education and higher learning on Earth, and with a solid understanding of the storyline, he was still able to uncover a lot through careful observation of the physical changes his body underwent while using his powers, along with mutual experimentation and discussion with Enel.
Naturally, since both of them were Logia-type users, their discoveries revolved around the nature of Logia Fruits.
For instance, Yemu realized that the ability of a Logia-type Devil Fruit to generate its respective element wasn't infinite.
Take his Mera Mera no Mi as an example—while he could produce flames, that didn't mean he could create endless fire at will. It felt more like there was a substance within his body fueling this process.
Based on descriptions from the original story, Yemu gave this substance a name: Lineage Factor.
This Lineage Factor could convert the energy from his physical body into flames. The more fire he generated, the more energy it consumed.
This also explained why those with stronger bodies often exhibited more powerful Devil Fruit abilities.
At first glance, this might seem like a drawback. But on the contrary, it showed that the dominance of Devil Fruits wasn't absolute. As long as the user was strong enough, they could completely override the typical weaknesses and limitations.
It opened a path for Devil Fruit users to evolve, and perhaps even awaken their abilities.
Another key insight was about elemental transformation. Yemu discovered that Logia users didn't become elements to simply dodge attacks, but rather to manipulate their body's flow and transformation.
Put simply, Logia users could turn their bodies into elements and move freely within those elements, like water flowing through itself.
This might sound obvious, but it confirmed something critical: Logia users don't become intangible or disappear—they still have a physical form. It's just that their body behaves like a fluid, hidden within the element, making them impervious to regular physical attacks.
This also explained why Haki could hurt Logia users.
Those trained in Observation Haki could pinpoint where a Logia user's real body was while in elemental form, and then use Armament Haki to strike that spot.
It could also explain why some Logia users don't fear Armament Haki—if their true form couldn't be located, then even Haki couldn't reach them.
As for how an Armament Haki user could catch a moving, flowing Logia user—Yemu didn't know yet, since he hadn't awakened Armament Haki himself.
But he had a theory. There are only two ways to catch water: freeze it, or use water to trap water.
Now, if the two points above described traits common to most Logia Fruits—excluding the Yami Yami no Mi, of course—then the next insight Yemu had might apply only to specific types, like the Mera Mera no Mi, Goro Goro no Mi, and Pika Pika no Mi.
These three fruits all shared a unique trait: their elements—fire, lightning, and light—were composed of pure energy. This made them fundamentally different from other Logia elements like ice, magma, sand, or smoke, which all had tangible, touchable forms.
This not only made these energy-type Logias harder for Haki to lock onto, but also sparked a new idea in Yemu's mind.
An idea closely tied to his desire to develop the Rokushiki techniques himself…
…
Since he was now in the world of One Piece, it only made sense to master the Rokushiki as early as possible—especially before unlocking Haki. After all, there were rumors that the Rokushiki was the foundational form of Haki.
But in the end, Yemu gave up on the idea—not because he couldn't do it, but because after thinking it through, he realized it just wasn't worth the time.
The Rokushiki consists of six techniques: Soru, Tekkai, Kami-e, Geppo, Rankyaku, and Shigan.
But the more he thought about it, the more he realized that aside from Soru, which offered high-speed movement, the rest were redundant thanks to his Devil Fruit.
Tekkai and Kami-e were completely outclassed by his Logia-type elementalization. Rankyaku and Shigan? He could replicate those effects with his Devil Fruit. As for Geppo, he could already fly using the Mera Mera no Mi.
Enel had even less need for any of it. What was the point of Soru when his lightning was already faster?
In some ways, Enel's Goro Goro no Mi was even more versatile than Yemu's Mera Mera no Mi.
So rather than waste time on these somewhat redundant techniques, it made far more sense to invest that time in Devil Fruit development and physical training. And once they made it down to the Blue Sea, couldn't they just swipe the Rokushiki directly from CP9?
Still, even though Yemu decided not to develop the Rokushiki, it reminded him of one other ability that was far more intriguing—Seimei Kikan.
Seimei Kikan first appeared in Enies Lobby, used by that lion-like member of CP9. With it, he could control any part of his body at will—his hair, internal organs, limbs, muscles, and more.
Later in the story, Seimei Kikan also showed other functions—rapid digestion, stamina recovery, energy storage, even accelerating wound healing.
So Yemu had an idea—what if he could use a similar ability to modify his own body, making himself stronger from the inside out?
Even though his rigorous training had already pushed his body far beyond its limits, he knew full well he was still nowhere near the level of the true monsters out there.
The problem was, he didn't know how to use Seimei Kikan, let alone how to learn it.
But... he had the Mera Mera no Mi.
And that's where the earlier point about the Mera Mera no Mi, Goro Goro no Mi, and Pika Pika no Mi being special came into play.
All three allowed their users to transform into pure energy—the only difference was in how that energy manifested.
So Yemu wondered: could he tweak that ability, reshape it into a version of Seimei Kikan?
After all, for Logia users, while it was still uncertain whether they could convert external matter into their element, they definitely had full control over their own bodies—including the food they consumed.
With that idea in mind, Yemu spent the next three years experimenting, carefully and methodically.
Eventually, he succeeded.
He named this new ability: Nirvana.