"X-Axis" is Lille Barro's schrift ability, a power he received directly from Yhwach. The Schrift Yhwach bestowed on him was the letter "X," and that letter's meaning is precisely this power to pierce all things.
Lille habitually channels X-Axis through a reishi weapon that looks like a sniper.
The ability is terrifying: everything lying on the straight line between the muzzle and the target is pierced clean through, and, except for the Eight-Mirror Sword of the Ise clan, nothing can offset or defend against it. At least, that was true in the original story.
Lille often capitalised on his opponents' complacency, picking them off like paper targets.
Anyone struck by X-Axis suffers severe injuries, their body left with a perfectly round hole.
Because Lille himself tends to underestimate his foes, he hides his full power and thus limits X-Axis.
In the canon timeline, only after Lille was "purified" by Auschwählen and resurrected did he finally shake off those shackles and unleash his real strength: an invisible, purely conceptual shot.
That unseen attack even caught Royal Guard member Ōetsu Nimaiya off guard, running him through and dropping him to the ground.
Whenever Lille opens his left eye, X-Axis triggers automatically, his body firing rounds that pierce almost every life-threatening substance around him.
That alone shows how dreadful the power is. Worse yet, in this state Lille can perforate anyone he regards as a target, while any attack launched at him becomes completely ineffective.
It is, frankly, a cheat-level ability. In the source material, had it not been for Lille's personality flaws, even Shunsui Kyōraku would likely have fallen to him.
Lille's death always felt like the author forcibly writing him off.
Having sensed Hui's overwhelming strength, Lille decided, both to avoid being one-shot and to test how much power Hui truly possessed, that it was time to use X-Axis.
A moment later, the sniper rifle Diagram materialised in his hands, and without hesitation, he fired at Hui.
The reishi bullet shot forward.
Its speed was astounding, at least if one were an ordinary person.
To Hui, though, such an attack was laughably slow; dodging it would have been effortless.
"The last time you slipped away, and now you have the nerve to come back?"
With a casual swing of his sword, Hui sliced the bullet cleanly in two.
His gaze went ice-cold as he looked at Lille.
Seeing his attack severed, Lille's expression tightened. He had tried to overestimate Hui's swordsmanship, only to realize he had still underestimated it.
"Since you're so eager to die, I'll be happy to oblige." Hui said, keeping up his persona.
Hui kept talking, and with his Observation Haki, he locked directly onto Lille Barro.
In that instant, Lille's skin crawled. He felt as if he weren't merely being watched but seized—his whole body shackled, escape impossible.
'Come on, come on—break!!!'
Roaring inside his head, Lille's desperate will to live finally shattered the restraint and let him move again.
He wasted no time: calling on the power of the Shadow, he tried to slip away.
But Hui was already at point-blank range.
Lille could clearly sense the razor edge of the Zhi Yi; its killing intent was brushing his skin.
At the very last split-second, Lille vanished, reappearing in his hidden lair.
He collapsed to the floor in a heap, cold sweat streaming down his face.
When he wiped it away, he noticed a streak of red on his palm.
Hurrying to a mirror, he saw the thin cut the sword aura had etched across his forehead, and shuddered.
"Another inch and I would've been dead."
The scene replayed in his mind until his limbs felt numb.
He stood silently before the mirror, staring at his reflection.
Lille was a proud man, convinced he was Yhwach's supreme masterpiece, the being closest to god. That pride had bred a habit of underestimating opponents, keeping him from unleashing his full power.
Now that pride lay in pieces.
Had it not been for the escape using Shadow, Hui would have one-shot him.
"…Mochizuki Hui…"
Only after a very long time did Lille finally breathe the name.
Outwardly he looked the same, but a careful eye would see that his whole aura had changed.
"One last attempt. After that, I will report back."
He muttered to himself: a final probe to uncover more of Hui's strength.
Though Hui had nearly killed him, Lille still felt he'd reaped a huge reward, he'd discovered a new ability of Hui that could Lock-On to targets.
In that moment just now, the sense of being bound was unforgettable.
He'd truly felt Hui seize him with an invisible force. A power never shown before.
The discovery left Lille both awed and chilled, and made him take Hui even more seriously.
"My Reishi-body keeps me from dying outright, so next time I'd better enter Vollständig and fight him head-on," he told himself. "But I need every contingency in place; if I really do die, that would be far too stupid."
…
Back at the villa, Hui had no idea his brief clash had already jolted Lille into awakening early and abandoning his old habit of hiding his strength.
Hui had assumed Lille would head straight back to Wandenreich.
Yet after a long while he could still sense Lille lingering in the same spot.
"That guy… has he always been this bold?"
Hui muttered, half amused and half exasperated.
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