Start by Eating Jake Jake no Mi 89 The Lion King

In Masa's mind, there were four ways to develop the Neko Neko no Mi Model: Leopard. 

The first was awakening, which focused on developing his physical ability, recovery, stamina, endurance, and everything to the limit. It was like the jailers on Impel Down who felt nothing and quickly stood up even if they were beaten by Luffy and his group. While they had lost their minds, they were like immortals and would continue to stand up even if they were beaten down since their recovery allowed them to do all of that. 

The second was to strengthen a certain aspect of the Neko Neko no Mi, Model: Leopard, and the card of a leopard; then, he should either strengthen the speed, claws, leap, hunting ability, and many others. 

The third way was to strengthen the aspect of control, which allowed him to control all leopards. 

The fourth way, the last, was by evolving into the ancestral creatures within the line of leopards, which was related to feline creatures. 

Among all of those ways, Masa has achieved most of them, except in the first and fourth ways. 

The first might be strange, but as there were four Masa, his potential also grew four times stronger, and he knew that he hadn't hit his limit of potential, so he knew that he could still become stronger as long as he trained harder. 

However, it would take a long time for him to hit that potential since he wasn't a protagonist that would quickly grow stronger so suddenly, instantly comprehend all the complicated things as long as they fought a strong opponent. 

Yet, Masa needed to train daily, learning knowledge day by day, continuously until he grew into the way he was. 

He couldn't be like Issei, who could become the strongest creature in the DxD Universe in a year, Luffy, who could become the king of pirates in two years, or Tatsuya, who became the strongest being the moment he was born. 

For Masa, everything needed a process, and while he could become four times faster, he believed that as long as he faced those protagonists, their strength would increase dramatically, almost instantly, like a rocket. 

So, the best way for him to make the Neko Neko no Mi Model: Leopard was by using the fourth method, which was evolution. 

Fortunately, this was the world of DxD where all the supernatural beings existed, so in that moment, he didn't hesitate and tried to evolve this Devil Fruit into a Manticore type. 

"Gnh!" 

There was no rejection; everything was smooth, as he had thought, but the process was painful! 

The weak, normal Zoan-type Devil Fruit turned into a mythical Zoan type, Neko Neko no Mi, Model: Manticore. 

Naturally, due to Manticore being a feline type of creature, it was easy for him to reach an "awakening," or rather, it was so natural of him, and that was why he was glad that he had chosen this Devil Fruit. 

The innate ability of the House of Vapula greatly benefited him. 

In the beginning, he thought that this innate ability was just so-so since all he could do was to tame and control a feline, yet now, he was glad he thought. 

Still, he had to say, the difference between a manticore and a leopard was huge! 

'Moreover, this power!' 

Unlike the leopard, the manticore had various abilities, including fire breathing, a scorpion tail, toxin manipulation, and even wings. 

Still, this transformation was so painful! 

Masa was glad that he did all of this in his new laboratory since he didn't want to make anyone worried. Rubbing his temple, he took a deep breath and decided to eat to recover his stamina. 

"This is troublesome..." 

As he grew stronger, naturally, his energy consumption also increased. 

Probably, in the future, he might also be like Luffy, who needed to eat so much that his belly would grow so huge, like obesity. While he could consume all of that quickly by using Seimei Kikan, or life return, it was expensive, and if possible, he wanted to gain energy in a different way. 

"Well, let's think about that later." 

What was important was to grow stronger first, and while his consumption was high, it was still within his tolerance, especially when he didn't lack money, and the food industry in his domain was also extremely advanced. 

Probably, in the Underworld, he controlled almost half of the food supply. 

Yes, food might not give him a lot of money, but it could give him a lot of power. 

While some devils might be starving, all the devils in his domain would be full, he thought. 

Nevertheless, what was important was still power, and because of that, he took all of the myth species of his lion creatures, including the Nemean Lion, which had been turned into Regulus Nemea, which had been tamed by Sairaorg. 

Yes, even though the Nemean Lion had long died, and its body had been turned into Regulus Nemea, its DNA could still be obtained as a Sacred Gear was an item created based on the combination of the body of the respective myth beings chosen by god. 

In other words, even if Regulus Nemea had formed into a certain item, as long as Masa picked this part of an item, it was similar to its DNA. 

Naturally, as this wasn't a complete DNA, Masa needed to compensate this DNA with something else since, unlike manticore, sphinx, shisa, and many other mythical lions that existed within his domain, so the difficulty was even greater, the Demonic Energy had a limitless possibility. 

Yes, while the amount of Demonic Energy had been determined from birth, there were many things that one could do with it, including to help him with this transformation. 

Moreover, along with the knowledge of Vepapunk about biology and medicines that was gained in the world of One Piece, Masa had decided to go all out. 

Still, how did he gain the DNA of the Nemean Lion, who had turned into a Regulus Nemean? 

Masa might have forgotten to explain this before, but after he helped Sairaorg's mother, Misla Bael, wake up, he asked for Sairaorg to give him part of the Regulus Nemean, which Sairaorg agreed to. 

Naturally, Regulus had no way to refuse, especially since it was a feline type of creature. 

If Regulus hadn't met Sairaorg first, he might have gone after Masa, it thought, at that time. 

"Let's do it." 

As Masa made up his mind, he transformed his Devil Fruit into a Nemean Lion. 

"Ghn!" 

He grounded his teeth, and as expected, it was so painful! 

Probably because the Nemean Lion was famous for its impenetrability, which made it impervious to all weapons, making it nearly invincible that his skin and body transformed, peeled in a grotesque manner, trying to transform his body to have a similar ability! 

Yet, he knew that he had to bear everything, and supported the healing process by using his Demonic Energy, life force, and many others until finally—

"...success." 

Masa could feel his defense had increased dramatically, and if he had added an Armament Haki, nothing could have passed through his defense! 

Probably, in this world, he was a creature with the highest defense. 

If he used the ability of Ton Ton no Mi and made his body denser, then there was no doubt that no one could pass his defense anymore, no matter what method they used, including the Power of Destruction! 

Still, he knew that he couldn't be arrogant, and playing defense wouldn't bring him to victory, after all. 

More importantly, even the Nemean Lion with the highest defense could die, so Masa knew that even if he had the highest defense in the world, he wasn't invincible, but before that—

"Masa, you are done with your training, right?" 

As the door was knocked, a cute figure was looking at his training room curiously before she fell into a panic. "Ah, Masa, what's happening!?" 

"R-Ravel... get me something to eat first." 

Yup, like Masa of Mahouka, Masa of DxD had a childhood friend, and let's talk about that later since he was so hungry!