Vector.
This concept frequently appears in mathematics and physics, according to Jay's knowledge.
It refers to a direction or orientation, and sometimes to the magnitude itself.
Hundred Eyes uses its main eye (the power of the vector) as an ultimate protective ability, but it can only apply it within a specific area of effect.
A wise choice, according to Dōma… but also a foolish one.
Why such an assessment from Dōma?
"The power of vector control is formidable, but Hundred Eyes' limitation was a mistake born from a cruel lack of knowledge in mathematics and demon biology."
The vector was officially used for the first time in Europe during the 19th century, then introduced to Japan later.
Thus, around the 20th century, during the Meiji era, this concept began to spread in Japan.
Dōma currently finds himself in the middle of the Meiji era, as the Taishō era won't begin for another century.
(Author's note: Tanjiro lives in the Taishō era, and Dōma is at least a century ahead of his time. Thus, he belongs to the Meiji era, somewhere between 1800 and 1900.)
At that time, the vector remained a fledgling mathematical concept, as Dōma hails from an era straddling the 19th and 20th centuries.
Jay hadn't paid much attention to the details of the *KNY* world's timeline, but Japan during this period was only just beginning to open up to the Western world.
All these elements make Dōma a creature with vast theoretical knowledge in mathematics.
"The essence of the vector is 'direction,' but this element is applied to almost everything in nature," he says, gazing at his fan under the night sky.
In the *KNY* universe, demons often wield their powers through intermediaries.
Nakime, Dōma, Kyōgai, Gyutaro… the list goes on.
This choice is never made on a whim: it stems from logical reasoning.
Take Enmu, the future Lower Moon One, for example.
He marked train tickets with his blood, as he wanted to put Tanjiro and the others to sleep in a way that was both discreet and optimal. He didn't even need to use his power directly to achieve his goal.
Dōma and Nakime operate in the same way.
Their "weapon" holds their power, with their blood serving as a catalyst.
This is also why Gyutaro's sickle has that slightly "fleshy" appearance.
Dōma now observes his fan, as an idea takes root in his mind.
Why keep Hundred Eyes' eye in his body when he could imbue its properties into his fan?
This idea would solve the issue of vector control being limited to the body hosting the eye, while also preventing Dōma from ending up like Momoshiki.
It took him an entire day to achieve the optimal result with his twin fans.
Each of them now bears multicolored eyes, much like Kokushibo's blade.
Dōma still retains the powers of vector control and ice, like Enmu, but he can now use his vector remotely through ice.
He has created a perfect duo.
"Blood Demon Art: Miko."
A mini Dōma appeared after his invocation.
The ice creature looked around in confusion before fixing its gaze on Dōma and striking a military salute.
Unfortunately, it clumsily dropped its fan, resulting in a comical scene.
A cute spectacle… if one didn't know how terrifying this 1.2-meter creature could be.
"Back then, I created this creature inspired by the principle of a Wi-Fi connection and my understanding of Blood Demon Arts."
The Miko are not alive in the strictest sense but possess limited autonomy without Dōma's full control.
They are born and sustained by Dōma through his Blood Demon Art, but their actions follow a pre-established pattern.
They function somewhat like programmed software: a user (Dōma) can activate them with an "internet connection" (Dōma's Blood Demon Art) or let them operate in automatic mode, without information transmitted through the connection.
Of course, in the latter case, their efficiency is limited due to the lack of direct supervision from Dōma.
The Miko will only follow predefined protocols, receiving no additional information in the second case.
An example: its reaction to Dōma.
It's also thanks to this aspect of the Miko that Dōma could summon four of them without reducing their overall effectiveness.
The Blood Demon Art, *Paradise of Vanity*, wasn't the only reason Dōma could use four Miko without difficulty while managing the information they perceived and relaying it to them.
"Attack me!"
He didn't even need to say it, as the Miko was already receiving all the necessary information.
Dōma had said it simply because he wanted to.
The Miko nodded before lunging at Dōma.
Dōma waved his fan, but he realized he couldn't use ice.
Instinctively, he applied his vector control to the wind rather than his initial target.
After all, he was holding a fan.
*Crack!*
"Oh, nice parry… but you've lost…"
The small creature benefited from a reaction speed devoid of any hesitation, as it was free of emotions like fear.
It struck with its own fan, generating a wave of ice made of tiny crystals, but Dōma had created a blade of wind more powerful than expected.
The Miko was hurled into the forest, only stopping after breaking a few trees.
'It's indeed powerful… but not invincible.'
Dōma had solid defense and attack, as he retained the original properties of the vector power.
Yet, a problem persisted in this exchange: he was unable to apply the ice power to his fan when using the vector effect.
If Dōma wanted to combine ice and vector, he had to use one fan for ice and the other for the vector.
Another solution would be to let the Miko use ice while supporting it with vector control, or vice versa.
"Other than that, my body seems incapable of using the vector at the moment of attack…"
The vector could only be used in one domain at a time.
A situation similar to Accelerator from *Toaru no Index*.
…
The applications Dōma found for vector manipulation that night were numerous.
He mentally classified them into three categories.
1. Support Abilities
The most significant discovery concerned environmental manipulation, which Dōma had refined through his training.
He glimpsed the future possibility of slowing particle movement to absolute zero, the complete cessation of all atomic vibration.
For now, he was only amplifying the cold generated by his ice to an extreme degree.
The goal was to create a freezing field as vast as during his fight against Hundred Eyes, and in mere seconds.
However, Dōma had to rely on the help of his Miko, or the ice priestesses summoned by his Blood Demon Art.
The cold spread so quickly and violently that he estimated he could crush most opponents simply by altering the climate around them.
Blood Demon Art: Eternal Blizzard.
This mastery of frost wasn't his only discovery.
He had also developed another technique to counter the sun itself:
Blood Demon Art: Summer Mist.
A fog so dense it obscured vision, mixed with ice crystals capable of necrotizing the lungs of his enemies.
With these two arts, Dōma didn't even need to strike: he simply had to buy time and prevent any escape.
2. Physical Abilities
Vector manipulation also made the impossible possible on a physical (bodily) scale.
For example, shattering a colossal boulder with a single swing of his fan.
Normally, Dōma wasn't a brute-force fighter like Akaza, but by continuously altering the vector of his own speed, he layered accelerations.
At the precise moment of impact, it was as if gravity itself or an invisible force propelled his strike forward.
Blood Demon Art: Force of the White Oni.
The result: terrifying kinetic energy.
3. Defensive Abilities
Finally, the simplest category:
Blood Demon Art: Vector.
Within a precise radius, Dōma could redirect any movement or force.
A sword attempting to strike him?
He simply returned the vector of impact to its point of origin, breaking the blade if the swordsman lacked skill.
Far more subtle than Hundred Eyes' approach, which merely deflected attacks without ever redirecting their power.
The only weakness of this technique lay in its limited area of effect and its inability to be used simultaneously with other vector techniques.
"Hmm~ I've worked hard…" he murmured, as the night sky began to show the first glimmers of dawn.
He had laid the foundations for his new techniques, but Dōma still needed time.
These arts had to be mastered and refined to fully integrate into his fighting style.
Dōma still had much to accomplish.
Unfortunately, the world wasn't about to give him time to himself.
"Lord Dōma… would you like to come with me to this year's festival?"
Kikyō's voice rang out as he crossed the monastery's entrance.
Dōma was greeted by a visibly nervous Kikyō.
The young priestess didn't even greet him properly before immediately proposing he accompany her to the festival.
'I knew it… she's overwhelmed with work lately… *sigh* It looks like I'll have to help her.'
The ice demon misinterpreted the young priestess's intentions, as the sect was organizing several stalls and activities for the New Year.
Normally, Dōma no longer concerned himself with these tasks, having delegated his responsibilities across the sect's various departments.
However, he had established a branch of the cult in another city, and the lack of manpower was starting to show.
This shortage translated into Kikyō's recent clumsiness.
Dōma thus concluded that the young woman needed his presence to further promote the sect during the festival.
All these elements turned poor Kikyō, despite herself, into the organizer of a work-related outing rather than a romantic date.
'Tomorrow, I'll have an entire night with Dōma-sama… !'
The poor girl still had no idea she had been completely misunderstood.
***
Author's note: I should create a character sheet based on Dōma, a kind of detailed status.
If you're okay with it, I'll publish it in the appendix tomorrow.