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“If you keep eating demons regularly, your stomach isn’t going to be able to handle regular food anymore, the same way a human’s stomach shouldn’t be able to handle eating a demon.” Shinobu had warned. “So you need to only resort to that if your life is on the line.”
But being a Demon Slayer means your life is almost always on the line, especially when you don’t have big, fancy breathing styles to ward off your enemies.
Every time Genya eats a demon, it takes a little longer for him to return to his human form, and it hurts a little more. That’s the main side effect. Shinobu theorizes that eventually he will be unable to turn back at all, and he will be stuck as a demon.
The funny thing about demon flesh is that there is absolutely nothing nutritious about it. It turns to ash in Genya’s mouth, usually, and he basically just absorbs something inside it which allows him to half turn into a demon himself. It doesn’t ever truly make it to his stomach, and if it were to, he would not digest it in the traditional sense.
It doesn’t take many years of being a demon slayer for him to start having trouble with food. It isn’t very obvious at first, and he mostly chalks the nausea up to his constant activity, or it being spoiled.
But that doesn’t work forever, and one day he gets truly sick.
It’s right after a mission, when he’s having dinner with Gyomei. It’s tradition that whenever the two of them are home at the same time, they share at least one meal a day. Genya loves it.
Gyomei cooked tonight, since Genya is the one returning, and he prepared one of Genya’s favorites. When he sets the food on the table, Genya’s stomach turns. His smile wobbles a little, hand reflexively meeting his middle. He hadn’t felt sick before now, so he brushes it off.
“Thank you,” he says. Gyomei nods and offers a smile of his own. Genya’s stomach continues to squirm as they begin to eat, and swallowing becomes a chore. It tastes more like dirt than food in his mouth, and he thinks he very well may throw up. He doesn’t want to say that, though, and risk hurting Gyomei’s feelings, even though he knows the Stone Hashira will not be offended. “Uhm, I’m sorry, Sensei, but I’m not feeling well.” He admits, bringing his arms around to hug his now aching middle.
“That’s alright, Genya,” he replies, softly. “What’s the matter? Would you like to go lay down?”
“I think that would be best,” Genya breathes, nausea growing as sweat breaks out across his forehead. “I think I’m going to throw up.” His voice is practically a whine, at this point. Gyomei looks concerned and moves to help Genya up.
“Alright, let’s get you to bed, then,” he offers, putting one of his large hands on Genya’s back.
From that point on the night goes sour. Genya cannot hold down anything he’s given, not even water or the light broth Gyomei makes him. He has no fever, no other symptoms, but anything he swallows comes straight back up. It’s scary. By the next morning, when he hasn’t eaten in well over twenty four hours, Gyomei decides to take him to Shinobu’s.The woman never stops smiling, but the worry in her eyes is very clear.
“Do you remember when I warned you that eating demons would have adverse effects on your body later down the line?” she asks, pulling a stool over to Genya’s bed in her infirmary/ He has an IV tube in one arm, giving him some sort of fluids. He nods hesitantly. “Well, these are those effects. You can’t eat food anymore. Your stomach can’t digest it, and so it rejects it instead.”
Horror dawns over Genya’s face as he squeaks out a quiet; “What?”
“I’m sorry,” says Shinobu.
“What do I eat, then?” Genya asks, clutching his hands together in his lap to keep them from shaking.
“That’s tricky,” she replies, uneasily. Unease is a weird thing to see on her, too, as she’s usually not very emotive. “I’d say demons, but they don’t actually feed you. They aren’t nutritious, so eating them is pointless beyond the boost it gives you.” She hesitates for a moment. “So, I think that really the only option would be… human blood.” To Genya it feels as though the world stops turning.
“No,” he laughs, a little hysterical, completely humorless. “No, I’ll just- I’ll just stop eating demons and go back to normal. If I just keep eating real food I’ll get used to it again. I’m not- I’m not a demon.”
“You’re not,” Shinobu agrees. “But this isn’t something that can be fixed. You’re not going to have to hurt people to get it, or anything, there are ethical ways to obtain it. I know how it sounds, but I promise, you’re still a person.”
But is he? Genya has always wondered at what point the line between human and demon becomes so blurred he can no longer distinguish it. Is this the line? Has he gone so far he cannot return?
“It’s alright, Genya,” Gyomei says, resting a hand on each of Genya’s shoulders. “It will be alright.” For the first time, Genya doesn’t believe him.
It’s interesting how awful blood tastes. It’s like liquid metal, but doesn’t really have a distinguishable flavor beyond that.
To Genya, now, it’s like Heaven. It’s sweet, if still a little metallic, with something else he can’t describe laced through it. It hits his stomach like a hearty broth, and only now does he realize how hungry he had truly been.
He can feel eyes on him as he greedily drains the glass of it, mind completely blank as he fails to savor the amazing taste. He feels so guilty for enjoying it, but oh, it’s amazing. He wishes it wasn’t but it is.
Someone takes the glass from him when he is finished with it, and they all wait with bated breath to see if he can keep it down. His stomach remains settled, satisfied, even, as the guilt settles in. They’re trying not to, but Gyomei and Shinobu both look mildly disgusted, and they’re almost certainly going to hate him, assuming they already didn’t.
“Well, how do you feel?” Shinobu asks, tapping her pen against her paper, ready to take notes.
“Uh, better, actually,” Genya admits, sheepishly. “I don’t feel sick. I think it worked.” It almost hurts to say. It’s shameful, absolutely foul. He feels the need to apologize, but he thinks it won’t change anything if he does.
“Good.” She scribbles something on the keyboard. “It could be interesting to run some tests on you, see how else you change over time. I wonder if different blood types taste different to you.”
“M-maybe,” Genya says, trembling. He feels disgusting, but, thankfully, not as though he’s going to throw up.
“That can wait until another day,” Gyomei interjects. “I think that we’ve all had enough excitement for one day. I’m sure Genya is exhausted, and taking him home would be a good idea.” She nods, though looks a little disappointed.
“Fine,” she sighs. “I’ll send you off with a couple of the blood bags I have on hand, for now. If anything seems off, come back immediately. Vomiting, fever, anything.”
“Of course,” Gyomei guides Genya to the front of the Butterfly Estate, a little tote of the blood bags clutched to Genya’s chest. He thinks he can smell the blood through them, but he’s probably imagining it. Either way, he’s still rather hungry, even if it’s not as bad as it was earlier. “Thank you, Shinobu.”
“Yeah, thank you, Miss Kocho,” Genya says. She nods, and waves them off. “I’m… I’m really sorry, Sensei.” Genya finally murmurs, once they’re a sufficient distance from the Butterfly Estate. “I know it’s awful, and disappointing-”
“Do not worry about it, Genya,” Gyomei hushes him, pulling him into his side. “I’m not upset with you. It will all be ok.” Genya sniffles, and Gyomei gently pets his hair. “You’ll be ok.”