So, when I first laid my eyes on Kaigaku, I had no opinion of him other than: 'that nasty bit o' jerka**'

About two weeks later, here I am, writing a fic about him. Is this some sort of Blood Demon Art-induced hypnosis?

Notes: Zenitsu sometimes refers to Kaigaku as 'aniki' – which can both mean elder brother or a senior. I heard that women do not use that word often but since Zenitsu is using that word in place of 'senior', I've just kept its usage.

Female version of Zenitsu retains the same name. She might sometimes be called 'Zenko', but only from time to time.

Zenitsu here is aged up by a year or so. (Fortunately, Zenitsu is canonically 16, older than his friends - so not 'too' much of a difference.)

Warning: Lots and LOTS of manga spoilers up to latest chapters.

There's no shame in trying to survive.

Ever since he could think for himself, it had been Kaigaku's steadfast principle.

If that means becoming something other than human, becoming the very being that he'd been trained to annihilate, then so be it.

It takes a while for a Breath User to become a demon.

After imparting that information, the Upper Moon No. 1 had left the now-former hunter to his own devices, unwilling to waste time waiting until he became a full-fledged demon. Kaigaku had been grateful for that. He wasn't sure he could stand the powerful demon's presence for more than he'd already endured.

It'd already been three days.

The first day had been spent in agonizing pain. As Muzan's blood bestowed upon him raged inside, seemingly rearranging all his cells.

When the pain was gone, he could feel something had changed. He's grown averse to sunlight, which was a concrete proof of transformation. There was a frustrating restlessness in his body as if each of his muscles had become separate creatures. But no burst of power as he expected. No thirst or hunger even when he'd gone two days without consuming anything. Although he felt slightly dazed, like his brain was wrapped in a cotton wool. Was this normal? What if something had gone wrong...

Perhaps he only became a demon properly when he had his first taste of human blood.

But really.

"Kaigaku?"

To think he'd run into this idiot first. She hadn't changed at all since he'd left. Still puny. Fidgety. Like a harassed chicken.

"Wha – Weren't you out on a mission? Uh, did you come back for a break?"

Of course, he'd been wandering aimlessly and his feet had taken him upon the familiar path. And with his perception so dulled...

"Or – is your mission – here? Does – does that mean there are demons in this area?!"

Ha, you're not wrong about that.

The blond girl glanced around warily as she walked over to him. Unfortunately, in trying to spot any demonic presence, she failed to spot a protruding tree root and fell over. Her haori fluttered open to show a torn slit at the lower garment.

Revealing a bloodied leg.

His heart gave such a thump at the sight that it was like being punched from the inside.

"You're bleeding."

Kaigaku murmured as if in a dream. The junior disciple looked down at her own leg, blushed, and tried ineffectively to cover it with her haori.

"Oh. Um, I did scrape my leg against something while I was -"

Kaigaku wasn't listening. He stared at the bright red streak along the dirt-specked skin.

And the world changed.

Everything came into an abrupt focus. As if a sudden tide had cleared away the previous haze. In the dark of the night, the moon was almost blinding to his newly-sharpened vision. He could hear the harsh breathing of his lowly colleague. (Ah, 'former' colleague.) Smell the metallic tang of her blood -

Then the hunger hit.

It was a claw that grasped the whole of his being.

He gasped and stumbled.

"Kaigaku? What - why do you sound like that -"

Through reddened vision, he saw the blond girl stand up.

That ridiculous tumble of yellow locks shown like a beacon under the moonlight.

Do anything to survive.

If it comes at the loss of others, so be it.

He didn't even feel himself move.

The next thing he was aware of was that they were both on the ground. With him upon her, the latter too stunned to even react -

His fangs tore into the bloodied thigh as easily as into a ripened peach.

Heat flooded his mouth. Assaulting his sharpened senses. Flowing throughout his body, turning the prickling restlessness into sheer power. He had never experienced anything this intense. He might have blacked out for a second for pure ecstasy.

A scream brought him back.

Scrawny arms pushed him away with surprising strength. He didn't resist. Too drunk on the blood to care. The force of the push tore a sizable chunk of flesh off the leg he was latching on. He greedily chewed the delectable morsel. He could imagine how he must look like, from the almost comical look of horror on the girl's face. His wet mouth broke into a grin. Lengthened teeth bit into his lower lip. He felt the trickle of his own blood as his nails - now more akin to claws - slashed his palm open. Exhilaration coursed through as those wounds immediately closed up and healed.

He had never felt this powerful. Invincible. Immortal.

So this was what it meant to be a demon.

He couldn't believe how he'd survived until now as a human.

"You're useful for once, Zenitsu."

Teeth chattering from shock, the girl fumbled for the sword at her side.

Seeing that, it suddenly occurred to him that the Final Selection for her was two months away.

He laughed at the stray thought. Honestly, he was surprised he bothered to remember.

She somehow did get the blade out. But by then, Kaigaku was upon her again, hauling her up easily with one hand. She yelped and grabbed onto his arm by reflex, the other hand doggedly clinging to the useless sword.

"Kaigaku! Don't – Let – GO! Gramps would - You – you can't -!"

"Don't you dare tell me what I can't do,"

Well, he'd be doing her a favor. It's not like she'd would survive the Final Selection. Better a quick, painless death by him. He'd offer her that much, as her senior.

… If she just kept still, just as she did whenever he bellowed at her. She blindly kicked and slashed at him, screaming all the while. In terms of damage, the latter was infinitely more effective. Kaigaku grimaced at the impact of each high-pitched crescendo seemingly piercing all the way into his skull. Wasn't she supposed to have keen hearing? It was a wonder she didn't kill herself with her own hollering.

He harshly pushed her head to the side and yanked down her haori. (Same pattern as their master's, he never could stand that either.) That earned him a blow to the side of his face with the butt of the sword – which actually managed to snap his head back. Kaigaku slowly turned to face the other – a familiar look on her, red and wet and puffy. But there was something that he had never seen before – in the set of the jaw and the eyes. Defiance. He was both impressed and irritated.

Zenitsu started another scream but it got cut off as the demon's mouth closed upon her neck. Kaigaku felt her freeze up. Which made the pulse beating frantically under the thin skin all the more pronounced. No wonder she whiplashed between being crazy and a state of near-stupor. With such erratic pulse. Just for a moment, he thought it a pity that this would still permanently once he sank his fangs in. He closed his eyes to savor that almost-pleasant throb against his lips while it lasted.

He narrowly dodged a blade coming from the side.

Clicking his tongue, Kaigaku stepped back, letting go of Zenitsu in the process. He spotted a man in the same uniform as his pull the girl back by the scruff of her neck.

"Are you okay? You're a student, right? Get back!"

"We heard about a demon sighting here. Thought it was just a rumor – "

"Is that – a Corps uniform?"

One of more practical members charged at him rather than asking pointless questions. Kaigaku evaded more out of habit. Unlike his fool of a colleague, these were official members in possession of Nichirin blades. But even they weren't a problem unless they cut his head off.

And right now, these low-ranking hunters seemed to move in slow motion for him.

He lazily knocked off the sword from another coming at him from behind. Then he grabbed and swung her in front of him. As expected, others hesitated facing the obvious human shield. Incompetents. No wonder the members other than the Pillars kept diminishing. To think he'd been one of them only a couple of days ago.

He sneered maliciously behind the thrashing woman. Then lowered his mouth towards her neck. Well, his former colleague can wait. She couldn't even move for fear behind the useless hunters. He could just whet his appetite before getting to her -

An abrupt bile rising up knocked the wind out of him.

What? He shook his head to get his bearings. Why –

He growled in agitation and tried again to bite into his captive's throat. Only to be hit by a stronger wave of nausea.

The Hell?!

He grit his teeth. Barely containing himself from vomiting right then and there. What was happening? It should've been the same as when he'd drank from Zenitsu -

On the third try, he'd almost clamped his mouth on the other's flesh. The repulsion was so great that he threw her away from him. He sank down on his knees, gagging.

He glared at the confused slayers dragging back their comrade. Even his vision had become blurry. But he could still make out –

One missing.

He lashed out behind a second too late. The crippling nausea got the better of him and the blade bit into his shoulder. Snarling more out of anger than any pain, he turned and twisted the arm that held the offending blade, forcing the hunter to drop his weapon. At the same time, his claws slashed at the hunter's neck, drawing blood.

Kaigaku didn't give himself time to hesitate and rushed at the other's throat. Even before his fangs dug in, the wounded flesh yielded blood to his mouth –

Kaigaku tore away retching. He dropped on all fours. This time, he did vomit, his whole body shaking from the force of it.

Making up in sense what he lacked in skill, the released hunter beat a hasty retreat. Calling out to his comrades:

"We won't be able to beat this one by ourselves!"

"But… that demon… doesn't look… doesn't act... normal…"

"… Do demons get sick?"

Kaigaku told himself that he'd kill the last one for that, even if he couldn't eat him.

He couldn't eat them.

He pulled himself up with effort, wiping his mouth. His uniform all bloodied from what he'd coughed up.

He glowered at the hunters. Food. All of them. He recited in his head experimentally, grinding his teeth for good measure. The resulting revulsion was immediate. Every single cell inside him rejecting the very notion. Almost causing him to lose consciousness.

"Oi, you keep back, you don't even have a…"

It was like a thinnest trickle of cold, pure water. Cutting through the nauseous haze and clearing the bile. Kaigaku gulped in the smell of that blood like a man on the verge of drowning trying to keep his head up. Heaving, he looked up to see Zenitsu push through the group of hunters, leaning on her apprentice's sword for support. Dragging her still-bleeding leg. Kaigaku felt his mouth open behind his fist, panting.

Fuck.

Fuckfuckfuckfuck. FUCK.

Kaigaku turned on his heels. His own body fought him, longing to be near that enticing smell of blood, that presence. He squashed that urge by sheer will.

Must get away. Away from her.

When he looked back for a brief moment, he caught a sight of the blond girl open her mouth – only to slap her hand over it. There she stood, shaking and muffling herself, until he turned away and was gone.

It was raining outside the cave.

Kaigaku curled himself up into a tighter ball.

Five nights. Six once the sun goes down.

He'd held onto the barest hope that his – 'condition' – would be temporary. Of course, deep in his heart, his very body – knew that it wouldn't be so.

For the past days, he'd tried to devour anything else – including wild animals, and even a demon. (A mayfly of one he'd come across by accident. The only damage it'd caused him was due to him trying to devour the horrified demon and getting the most terrible sickness yet. So great that the demon actually managed to run away.) All ending in failure.

He knew he was going to expire at this rate.

And his body wouldn't accept anything other than Zenitsu's blood.

He thought about kidnapping her. But how to keep her around? Dammit, he'd become a demon to be more powerful, and having to drag around a live human wouldn't help. He doubted the other demons – the Upper Moon especially – would be benevolent enough to turn a blind eye.

He wasn't even sure he could join their ranks anymore, as the Upper Moon had hinted previously. He will call you when needed.

Kaigaku now understood what he'd meant. Ever since he'd taken in Muzan's blood, he'd been aware of an inexplicable presence inside him – something propelling him forward, watching behind his shoulder. Forbidding him from ever speaking of HIM.

Ironically, he could clearly feel what it had been from its absence. It was gone from the moment he'd drank from Zenitsu.

He'd even tested it by speaking that name out loud. Kibutsuji Muzan. Nothing. He was still a demon, but he was – cut free, so to speak.

Now, Zenitsu had taken residence in him instead.

That was the worst. He could feel Zenitsu. Where she was. Whether she was sleeping or loitering about. Distance helped to diminish the presence, to the point where it settled into a weak buzz that just let him know she existed somewhere. But his whole being rebelled against being away so, pulling him back like a migratory bird following its inner compass.

He wanted to turn himself inside out and scrape out every bit of her blood that'd infected him, if that were possible.

Meanwhile, he had to avoid the Corps members. The previous group had brought in reinforcements but had failed to locate him. Right now, it looked like they'd figured he was long gone from the premises.

Which made him wonder, why hadn't Zenitsu inform them about him? For if she had, surely the search would be more persistent, and his master would have –

He pulled his legs in deeper into his gut.

He was no stranger to starvation. But he'd never suffered anything like this. His whole being pinpointed towards a single want. And the lack of fulfillment was driving him mad.

"Kaigaku?"

At first, he thought he was hearing things. Then the familiar voice pricked his ears again.

"Kaigaku! I know you're here! …. Somewhere,"

He must've been much weaker than he'd surmised, to not notice her approaching this close. He jerked his head up. The scenery outside confirmed his suspicion. He had not realized the sun had gone down, either.

"Would you just… Kaigaku - "

"Shut up, you moron."

Zenitsu whipped her head so fast that stray locks hit her own eyes. Kaigaku huffed in disdain as the girl let out a cry and staggered back.

He sat down on the ledge of the stone mountain where his temporary sanctuary was situated, looking down at the girl. It was half because he was too tired and half to refrain from flinging himself towards that delicious, seductive-smelling source of blood.

"What are you doing here?"

"What happened to you?"

There was a pause after the simultaneous questioning.

The girl was shaking from the cold. Trying to shield herself from merciless rain with her sleeves. Her miserable, pouting face proclaimed: 'THIS is how much my life hates me.'

Well, that was his line, at the moment.

Kaigaku was the first to break the silence.

"If you can't tell, you're more of an imbecile than I initially thought."

"I – I worked up a lot of nerve to look for you, you know!"

Kaigaku didn't even grace that with a reply. Now he knew that one thing bigger than her cowardice was her stupidity. Wandering this far out at night, knowingly looking for a demon.

As if she could do anything about whatever that had befallen him -.

He could see her breath mingle into a tiny mist-cloud in the night air. Could feel the warmth of her even from where he perched.

He dug his hands into the rock to keep his anxious body at bay.

"What are you doing here?"

The repeated question seemed to have gotten through the girl's thick skull.

"Let's - let's go back. We can talk to Gramps. I'm sure he can figure something out -"

He snorted derisively.

"What, like cutting my head off?"

"Can't you see I'm trying to not let that happen?!"

The sheer cheek of her. To think that she could 'let' anything regarding him.

"So why are you?"

Zenitsu hesitated. When Kaigaku growled impatiently, she blurted out.

"You - you didn't kill them. The Corps members."

"I tried to kill you."

He could practically hear Zenitsu gulp.

"Well - you didn't."

Kaigaku gnashed his teeth in a very demon-like manner. Zenitsu flinched but didn't back away.

"And you - sounded... panicked and... confused. Scared."

Kaigaku gripped the rock ledge so hard that a part of it crushed to dust in his hand.

"So - if you can just, just -"

"Why do you care? I know you hate me just as much as I hate you."

Her already pale face nearly turned blue. Then she hung her head so those preposterous ginkgo locks hid her face.

"But you're - Gramps's successor... And, and, you're my aniki -"

"I'm not your ANYTHING."

Zenitsu cowered with a hand over her head, as if the sheer venom in his voice had physically hit her.

"Please, Kaigaku- think of Gramps. He'll - he'll take care of this. I -I'll help too -"

Without any memory of having crossed the distance between them, Kaigaku found himself with a hand around her throat. Her gasp was squeezed into a choking gurgle as he slowly lifted her.

"Help? HELP?! YOU?"

The sword at her side clanged against him when he shook her. He despised that almost as much as the haori she wore. Despite the whining, despite the escape attempts, despite everything -

She was never without a sword at her side.

"You've been NOTHING but HINDRANCE."

He'd have hated her less if she'd given up. If she hadn't learned the First Form.

To have the gall to learn it when she couldn't even use it in an actual battle.

"Do you know why I became a demon in the first place? Do you?!"

Heedless of hands desperately scratching at his arm, he tightened his hold.

"It's because of you. It's all your fault. If you hadn't - been -"

'Kaigaku, after the Final Selection, you and Zenitsu will both be my -'

And now, to be tied to this pathetic slip of a girl forever? What had he done to deserve such fate? He tried. He trained so hard - trying to survive -

He'd rather -

"I should devour both you and the old man and be done with it - "

Brown eyes flared open. Flailing hands grasped the demon's wrist in a sure grip -

Lightning struck at that moment.

And Zenitsu was flung into the air.

For a second, Kaigaku just watched the yellow form float through the lighted sky like a leaf. Strangely detached. The whole scene seemed to play in slow motion, something not real.

He only came to his senses as Zenitsu was about to go over the edge of the ravine.

Her body would shatter, at this height.

Shit. SHIT.

Kaigaku lunged. Kicking off the ground to gain speed. He reached out for her but it looked like she was going to smash into the opposite cliff in her descent -

The girl spun, seeming to spring off from the air itself, landing on her feet perpendicular to the cliff side she'd been about to crash into.

With her hands on the sword.

"Thunder Breathing. First Form."

And Kaigaku was completely unguarded.

"Thunderclap and Flash."

Lightning split the sky once more.

As thunder rolled in belatedly, Kaigaku blinked at the spinning night sky.

It can't be.

From the corner of hectic vision, he saw his body below crash down onto the rocky ground, spurting blood from the neck. Which ended in a stump.

If it'd been a Nichirin blade, he'd be dead.

She cut me? She managed to - that worthless piece of –

Impossible. That wasn't right.

He - was his severed head him? - dropped next to his body. The fall hurt more than the cut. How fast it must've been for him to not see, let alone feel -

The skewed sight of the girl crouched in the finishing stance greeted him.

Rain thudding upon her body fizzed to steam. She shuddered.

And snapped her head up with a cry.

She stood frozen for a moment. Took a step back. Realizing how dangerously close she was to the ledge, she jumped back screaming, crawling away on all fours.

Then she turned and came face to face with Kaigaku.

She looked back and forth from the head laying on its side to the bloody body next to it.

The howl that emitted from her mouth drowned out even the thunder.

"WAAAAAAAH! Kaigaku! What - You're - you're not - don't tell me -"

Trembling, she stretched out her twitching fingers. When the head scowled, she fell back on her haunches, bursting into tears.

Are you KIDDING me.

"What - what, what happened?! Didn't you - throw me - "

"What's wrong with you, dumbass?"

"Aaaah! You, you're talking! But your head, I mean, you're a, a, a head..."

Bewilderment took over anger and humiliation. Kaigaku sighed and grit out the next words.

"Just. Get. My. Head. Back. On."

"Um, oh! Ah..."

Zenitsu gingerly put her hands forward, shuddering all over as fingers brushed his hair. Another yowl erupted when she accidentally touched the cut stump. 'There's blood! It's all over my hand!' 'Of course there is! Whose fault is that?!'

She lifted her senior's head, trying not to make eye contact. Just before moving towards the body, she faltered, and spoke. With her face still turned away.

"I'll, I'll, I'll - only do it if you promise to not hurt Gramps." Then she added almost as an afterthought. "And... maybe not eat... anyone...?"

Rage fleeted over the demon's features. Only to dissipate.

After a moment, the head began to wobble. Astonished, Zenitsu stared.

Kaigaku was laughing.

"I can't."

"What?"

"I can't eat anyone."

His voice quivered between a laugh and a sob.

"I can only consume your blood."

Kaigaku had always known he was special. And he was.

Just not in the way he'd expected, or hoped for.

End Note: That was… a much longer setup chapter than I initially thought.

I think the happiest part about this AU is that Gramps lives.