Mei Terumi possessed two Kekkei Genkai, and from the original story, it could be vaguely understood that someone like her, with two Kekkei Genkai, was likely the first in history.
There were no more than two possible explanations for the origin of such dual Kekkei Genkai.
The first was that she had been born with them, inheriting one from each parent and possessing extraordinary talent.
Although this possibility was extremely low, almost like a legend, it was not zero.
After all, Kekkei Genkai were passed down to descendants through genes. If Mei Terumi's parents each possessed one Kekkei Genkai, it was not impossible for her to inherit both.
However, there had surely been quite a few examples of people with two Kekkei Genkai marrying and having children throughout the history of the shinobi world. Yet there seemed to be only one example like Mei Terumi, which was why I said this possibility was extremely low.
The second possibility, and the one I personally considered the most likely, was that she had learned them later in life.
We all knew that Kekkei Genkai Ninjutsu could be obtained through learning. For example, Rosshi's Lava Release had been cultivated with reference to the Four-Tailed Monkey's Lava Release. Rosshi's information explicitly recorded that his Lava Release had been acquired through later study.
There was also Ōnoki's Dust Release. This was a ninjutsu that fused three types of Nature Transformation, one level higher than the fusion of two types required for a Kekkei Genkai. Even a ninjutsu like that could be obtained through learning.
Therefore, obtaining a Kekkei Genkai one level below it through learning was practically a certainty.
It was just extremely difficult.
For someone like Kakashi, specializing in Lightning Release was already enough. There was no need for him to study a Kekkei Genkai. As for whether he could actually master one, that was hard to say.
Then there was Orochimaru, a genius of ninjutsu recognized by everyone. Did he lack the talent to practice a Kekkei Genkai? If not, why did he not possess one?
The answer was that there was no need.
Everyone had things they were good at and things they were not. For example, Kakashi excelled at Lightning Release, and his compatibility with Wind Release, which countered Lightning Release, was certainly poor. Studying Wind Release would surely have taken a great deal of time and effort.
Instead of focusing on his own Lightning Release, why would he waste his energy studying something else? Wouldn't that just be pointless?
Thus, everyone's circumstances were different. Combined with the difficulty and high threshold involved in practicing a Kekkei Genkai, this resulted in most Kekkei Genkai in the shinobi world being inherited through clans, with very few people learning them independently.
Therefore, I personally believed that the possibility of Mei Terumi acquiring her Kekkei Genkai through independent study was extremely high.
But whether she had been born with one and learned another later, or had learned both of them later in life, was difficult to determine.
However, learning two later in life would certainly require more time and effort than learning one.
Considering that she had missed the best age for marriage because she had ended the dark era and rebuilt Kirigakure, this showed that her work must have been extremely demanding, leaving her with little energy to devote elsewhere.
I leaned toward her having learned only one and being born with the other.
Therefore, in my own setting, she would have Boiling Release in her youth and later learn Boiling Release. (Arguing with me again about which one she was born with and which one she learned later would be going too far!)
As for why she had gone to such lengths to learn a second Kekkei Genkai, I had a preliminary idea. If I got the chance, I would write it into the story later.
Here, I also had to clarify the matter of Boiling Release: what she possessed was "Boiling Release," not "Lava Release"!
This was not a typo on my part!
In Kishimoto's setting, Mei Terumi possessed Boiling Release, while Rosshi and Kurotsuchi possessed Lava Release.
We all knew that Japanese people used Chinese characters too. Even we Chinese could confuse the characters for "dissolve" and "melt" if we weren't careful, so it was hardly surprising that Japanese people might confuse them as well.
When Mei Terumi appeared at the Five Kage Summit, the Boiling Release she used was a field of bright-red magma that looked like Lava Release. In reality, this was a mistake by the anime production team.
Note that I was not making this up. If everyone searched for "Boiling Release" and "Boiling Release" on Baidu, they would find special annotations above them.
Because Kishimoto forgot to communicate with the production team, they thought he had written the wrong character and made Boiling Release's effects look like Lava Release.
This mistake was not corrected until the Five Kage's battle against Madara.
When Madara used Wood Style: Deep Forest Emergence, Mei Terumi used Boiling Release once. That time, its effect was a milky-white, sticky liquid. That was Boiling Release's true appearance.
Therefore, Boiling Release and Lava Release were different things. I had not confused them or mistyped the characters.
Mei Terumi's Boiling Release was the product of fusing Water and Earth natures: a milky-white, sticky liquid with rapid solidification and powerful corrosive properties.
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