Dôma.
In the manga Kimetsu no Yaiba, he is the Upper Moon Two and the third most powerful demon.
According to Jay's memory, some fans speculated that he could have single-handedly saved Muzan if he hadn't devoured Shinobu Kocho.
Others even believed that, given his rapid growth in just a century of existence, he might eventually surpass Kokushibo.
A logical assessment, as Dôma's laziness and disinterest hadn't stopped him from outmatching the power of all demons older than him, save for Muzan and Kokushibo.
All of this speaks to his genius, despite his mental issues and… peculiar habits.
Why bring this up now?
'Can I really win this sparring match?'
That question surfaced after Dôma arrived at a training ground reserved for demon battles within the Infinity Castle.
The idea that an Upper Moon Six could defeat the Upper Moon One seemed absurd… and yet, Dôma was pondering it.
The demon wielded vector manipulation for defense, ice to control and attack his enemies.
Above all, he possessed the "Vanity Paradise" to handle all tasks without being overwhelmed by options.
Thus, the idea of victory slowly took root in his mind.
Yet… that notion was quickly swept away by logical reasoning in his head.
Dôma wasn't the strongest… at least, not yet.
'Muzan is cautious and cowardly by nature. It wouldn't be wise to show too much growth too quickly…'
He decided to face Kokushibo seriously while preparing to abandon victory at a critical moment.
That way, he could learn from this sparring match without offending the pride of his superiors.
"Are you… ready?"
Kokushibo's calm voice echoed through the Infinity Castle as he slowly drew his blade from its sheath.
Dôma's reflection was perfectly visible on that demonic blade, embedded with eyes.
Kokushibo's sword didn't intimidate Dôma… nor did it make him hesitate, but only temporarily.
"T… this aura…"
An intense fighting intent radiated from Kokushibo as he assumed his combat stance.
Dôma felt as though he were facing seasoned warriors who had lived through countless battles.
'N… no, this isn't just a feeling…'
From the Sengoku era to the current Meiji era, several centuries had passed.
During all that time, Kokushibo had never slackened in his sword training.
The silhouette of "that man" had never stopped haunting his mind since their last encounter.
Kokushibo had fought and killed many demons foolish enough to covet his position and eliminated every demon slayer unfortunate enough to cross his path.
All these factors had forged this unique aura that even Muzan didn't possess.
One was a warrior guided by codes; the other, a monster devoid of true principles.
That was why Muzan's impression on Dôma during their first meeting differed from the one Kokushibo exuded now.
One wanted to kill… the other simply wanted to prove his superiority.
Dôma unfurled his two fans, displaying his first genuine emotion since meeting Kokushibo:
Excitement.
Dôma was excited.
He hadn't used his powers to provoke this feeling, but the same emotion he had lost at the end of his battle against Kamiya was starting to make his icy blood boil.
"We can begin, Kokushibo-sama~"
No sooner had Dôma spoken than he noticed Kokushibo's blade already aimed at him.
'S… so fast…'
Even surprised, Dôma's mind was far from ordinary, and the power of Vanity Paradise was working at full capacity to process all the information.
Dôma immediately used one fan to deflect Kokushibo's "casual" strike before employing the other to summon his miko.
'Blood Demon Art: Vector!'
"Blood Demon Art: Miko Crystallization!"
Four miniature ice Dômas were instantly summoned after Kokushibo's blade was partially destroyed by the vector effect.
The force of the blow had been turned back against the katana, but it didn't faze Upper Moon One.
Kokushibo paid no attention to Dôma's summons and slowly retracted his katana's blade toward himself.
The first attack had extended his blade toward Dôma, but the latter had countered it.
'He wasn't lying… this man is truly trying to counter his close-combat weakness… but did he use his full strength to break my katana?'
Kokushibo's mind analyzed all the information gleaned from this brief exchange as he "regenerated" his demonic blade.
Unlike Muzan, Upper Moon One was unaware of Dôma's vector power.
This led him to question whether Dôma might be hiding his physical strength.
A logical thought, since the ice demon was recognized as a strategist by both him and Muzan.
'I need to stop overthinking and simply defeat him for his insolence…'
Kokushibo decided to use absolute power against Dôma.
After all, what use was cunning in the face of overwhelming strength?
'I could just knock Dôma out to end this farce, but…'
Kokushibo thought back to his precious tea.
As Upper Moon One, he rarely went out and didn't feed on humans like other demons.
Usually, he sustained himself on the fools daring to challenge him.
Thus, tea was a rare culinary pleasure he indulged in as a "hobby."
'Indeed… he can't just walk away unconscious after this fight…'
Upper Moon One's six eyes gleamed with unmistakable resentment toward Dôma.
However, Dôma didn't have the luxury of paying attention to such details while facing Kokushibo.
"Oh… my hand is broken~ You're really putting all your desire to help me train into this, Kokushibo-sama!"
Yes, the hand he had used to redirect Kokushibo's force against his katana had shattered.
The reason was a millisecond's delay in using his vector power.
A result that could have been avoided if his power had been passive.
'Other than that, this monster didn't use any techniques tied to his breathing or Blood Demon Art…'
Yet Kokushibo had managed to push Dôma back and force him to summon his miko to handle any potential surprises.
Dôma smiled nonchalantly at the former samurai, but his mind was racing to find a way to bridge the gap in physical stats.
In the end, he managed to—but a little too late.
"Blood Demon Art: Moon Breathing, First Form… Black, Evening Palace!"
All Dôma perceived at that moment… was a sea of crescent-shaped blades.
…
"L… Lord Muzan…"
Nakime's usually monotonous voice carried a tone of urgency.
Muzan was in his quarters within the Infinity Castle. Before him, various laboratory equipment lay scattered… some cracked.
After Nakime contacted him, he had broken an expensive container that Dôma had provided using his cult's resources.
Yet Muzan didn't immediately show signs of anger.
The Demon King knew Nakime would never contact him first in the Infinity Castle without good reason.
"What's going on?"
Without hesitation, Muzan got straight to the point.
"… Dôma-sama and Kokushibo-sama are fighting."
At first, the refined, feminine form of Muzan's face remained impassive at the news, but that quickly changed.
After fully processing Nakime's words, Muzan spoke with an urgent tone:
"Take me there, now!"
No sooner had the request been made than the sound of Nakime's biwa echoed through the Infinity Castle.
Muzan found himself on a platform overlooking the arena where Kokushibo and Dôma's sparring match was unfolding.
The scene that met the Demon King's eyes was a spectacle he never expected to witness so soon in his plans.
"Cough… K-Kokushibo-sama… Y-you're really a tough one, aren't you?"