#demon's apprentice au

Tanjirou blinks. If you've read the manga, you know that demons don't blink because their eyes are continuosly damp for some reason. Tanjirou blinks out of habit lol.

Tanjirou has the size alteration ability, like Nezuko did. Though Tanjirou himself is unaware of this, since he uses it subconsiously. It takes him a while to figure it out.

Tanjirou shrinks down a little and slouches if he's in a situation where he disappear from it, mostly when he's uncomfortable and/ or unnerved.

Tanjirou straightens his posture and grows a little in height when he's in the prencense of someone he respects on some level, mostly when he's listening to Kokushibo's instructions.

Kokushibo is literally the only one who knows of Tanjirou's size alteration ability lmao.

Whenever Tanjirou is low on stamina, instead of getting hungry and possibly going feral, Tanjirou shrinks down to a child and gets drowsy, often also falling asleep. Kokushibo finds the sight odd and amusing.

Tanjirou's opinions on the Upper Moons and Muzan:

Muzan: "He's doesn't seem to like me that much. His precense is suffocating."

Kokushibo: "I respect and like him a lot. He's really nice and patient, and a good teacher. I don't want to disappoint him. His scent does sometimes get upset around me, but he never tells me why. He has scolded me for overworking before, but I don't know why he worries about that."

Douma: "His scent is weird... It's too empty. He unnerves me."

Akaza: "I saw him once, but we never talked. He did seem irritated upon seeing me."

Nakime: "She seems nice. I haven't talked to her much, but she's calm and quiet. Her Blood Demon Art amazes me."

Gyokko: "He looks interesting. He patted my head once as if I was a little kid because I stared at his pot for a moment."

Hantengu: "He cried constantly, even when I tried to comfort him. I didn't know what else to do. He's strong though, so I don't know why he cries and cowers a lot."

Gyutaro: "He called me a lapdog for no reason."

Daki: "She yanked my ear off and kept listing things about my appearance that she didn't like. She said that my eyes were pretty, and gouged them out just to look at them more closely."

Kaigaku: "He doesn't like me. I don't know why."

Tanjirou has total memory loss from getting turned into a demon. A few habits from his human life remain, but he can't remember anything.

Yes, Tanjirou still uses the "I'm the oldest" excuse when he gets told to stop pushing himself so hard. Kokushibo has scolded him for it lmao.

Douma has multiple times snuck behind Kokushibo's back to try and get to know Tanjirou more (to bother him).

Before knowing his name, Douma called Tanjirou "that cute little demon who follows Kokushibo-dono around". (Kokushibo was not amused by this.)

Right after turning into a demon, Tanjirou was in "feral newborn demon state" for about half an hour, which interested and confused Kokushibo, since he expected Tanjirou to be feral for much longer, weeks at most.

When Tanjirou is in the right state of his mind, Tanjirou eats very politely. Small bites, least amount of mess as possible.

Does Kokushibo know that he is starting to grow fond of Tanjirou? Yes. Will he ever acknowledge it? No.

Tanjirou absorbs information like a sponge. Parts of his Blood Demon Art is either inspired by or is a direct copy of other Blood Demon Arts. (Credits to Robby5428, who has made a book with a chapter helpfully explaining Tanjirou's Blood Demon Art, if you want to know more!)

Whenever Douma is in the room, Tanjirou tends to hide behind Kokushibo if he can. Kokushibo lets him do that without saying anything.

Don't try to pat Tanjirou's head if you don't outrank him or if he doesn't like you. He'll bite your arm off.

Due to being a demon and no longer limited by a fragile human body, Tanjirou tends to push himself to his limits over and over again. This worries Kokushibo a little, though he probably will never admit it.