Young Keita was convinced the culprit behind all this was a demon. With his sister's sixteenth birthday just around the corner, he had to find a way to save her—no matter what.
"Don't worry, Keita," said Mitsuri softly, her expression serious and resolute. "Your sister's going to be just fine."
They arrived at Keita's sister's room and found a girl about the same age as Mitsuri, though she looked like a shadow of herself. Her face was pale, her hair tangled and unkempt, and her whole demeanor screamed of someone at the edge of despair.
"Keita, who are they?" the girl asked timidly, poking her head out from beneath her blankets, her voice barely a whisper.
"They're here to help you!" Keita hurried to explain. "They said they can protect you!"
Like someone drowning who suddenly glimpsed the shore, the girl—whose name was Keimi—burst into desperate pleas. "Please, I beg you! Save me! I don't want to disappear!"
"Don't worry. We'll take care of everything," Mitsuri said gently, sitting beside her. Her voice had a soothing quality to it, like a lullaby that calmed storms.
With Mitsuri there, Keimi began to slowly calm down, her frantic breathing easing little by little.
"So your sixteenth birthday is tomorrow?" Yohan asked, his tone calm but firm.
"Yes… tomorrow," Keimi murmured, trembling as she spoke. The very mention of it seemed to shake her to her core.
"Then we'll stay with you tonight," Yohan declared. "Just try to rest. We'll handle the rest."
Keimi gave a weak nod, her strength spent. The constant anxiety of the past days finally gave way to exhaustion, and she drifted off to sleep.
That evening, Keita's parents returned home. Upon learning about the unexpected guests, they welcomed Yohan and Mitsuri with open arms. At this point, they had run out of options. Whether Yohan's claims were real or not didn't matter—any hope was better than none.
…
The room grew darker as night deepened. Midnight was fast approaching.
Mitsuri and Yohan sat silently on either side of Keimi, alert and poised.
As the clock ticked closer to twelve, Keimi stirred uneasily in her sleep. Her eyes flew open. "You'll really… protect me, right?"
"Just trust us," Mitsuri said with a soft smile. "Close your eyes. When you wake up, it'll all be over."
Keimi's eyes fluttered shut once more, but her trembling hands gripped the blanket tightly. Even asleep, she couldn't hide her fear.
"It's almost time," Yohan whispered.
Mitsuri nodded. "Yeah."
Their eyes met for just a second—completely in sync.
Then, it happened.
A shadow surged like spilled ink across the floor where Keimi lay, darkness swirling and congealing.
"It's here!"
Mitsuri moved in an instant, grabbing Keimi in one arm and slamming her fist into the ground with the other.
With a harsh screech, something inky and solid was struck and thrown back. Keimi gasped as she fully woke, seeing the writhing mass where she'd just been lying.
"That… what is that!?"
"That's a demon," Yohan replied coldly, shielding her behind him. "Stay behind me and don't move."
Out of the shadow emerged a pale figure with blood-red eyes and a single horn protruding from its head.
"So it's you…" Yohan narrowed his eyes. "The swamp Demon."
Mitsuri reached for her Nichirin Blade, the very same she had claimed during the mansion incident. Her customized weapon was still in the works, being crafted by the Demon Slayer Corps' blacksmiths.
"Disgusting," the swamp Demon snarled, baring jagged teeth. "Such delicious prey, and now you're going to ruin it!"
Its gaze never left Keimi, its hunger palpable.
"You dare… speak of people like food!" Mitsuri's anger ignited.
First Form: Unknowing Fire!
She slashed down with blazing speed, her flame-infused sword striking the demon's arm—severing it cleanly.
The Swamp Demon hissed, retracting into the shadows, clutching its stump.
"Trying to run?!" Mitsuri growled, preparing to chase it.
"Follow me!" Yohan called as he leapt out the window, carrying Keimi in his arms. They landed lightly on the rooftop.
Thanks to his Sage Body's heightened senses, Yohan could feel the demon's presence. "This way."
Mitsuri sprinted after him across the tiled roofs, fire in her eyes.
Eventually, the trail led them to a deserted street.
"You're not getting away this time!"
Yohan drove his blade into the wall.
A scream erupted.
The Swamp Demon's body slithered out of the stone, bleeding heavily.
"You filthy humans!" it snarled. "It's your fault! All your fault!"
Its bloodshot eyes locked onto Keimi again, but something had changed.
"She's sixteen already. The freshness is fading…"
"Rotten! Stale!" the demon howled in rage. "I'll kill you all!"
Then—more shadows surged up.
Two more demons materialized, one with two horns, the other with three.
"The full set," Yohan muttered. "One-horn, two-horn, three-horn…"
"All right, Mitsuri," he said calmly. "They're all yours."
"Leave it to me," she said, gripping her blade.
"Well, well," sneered the two-horned one. "Looks like we underestimated you."
"Let's change the menu. Let's eat this old hag instead!" the three-horned one howled with laughter.
"Old hag?!"
That did it.
Mitsuri's eyes went dark. She stared at them not like enemies… but like walking corpses.
She drew her blade.
Fourth Form: Blooming Flame Undulationl!
She became a whirlwind of fire, sword dancing like a living flame, cutting in wide arcs around her.
The two-horned and three-horned demons didn't even have time to scream properly before their heads were lopped off—reduced to ash without a single dying word.
The one-horned demon stood frozen.
Only one thought raced through its mind: Run.
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