Demon Slayer Headcannons

Jealousy

Tanjiro

- He probably doesn't know what it is to be honest
-"Why do I feel like this?"
-Honestly, overall he seems like someone who can give advice to someone else but never give it to himself
- He definitely move without thinking, so before he knows it, he's dragging you as far away as possible from the person he's jealous of 

Inosuke

- Emotions? He doesn't know how to deal with that
- "That person's face is annoying the hell out of me"
- The most confusion you'll see on a person's face
- As always, he never thinks before doing anything so he'll probably just sock the person in the face
- When you're being dragged away from the person, he says something like "I didn't like how they looked"
- And even though he doesn't know what he's feeling, you definitely know what it is

Zenitsu

- Crying
- "UWAHHHHH NOOOOO Y/N WHO IS THISSSSSS"
- Instead of being dragged away from that person, you're the one who has to apologize for Zenitsu being really loud and drag him away
- Later you scold him that he can't act like that every time he feels jealous
- Does he listen to you? Yeah because he loves you
- Does he ever actually do what he's told? No
-He's still crying every time he gets jealous

Sabito

- Confident that you'll stay with him but has second thoughts
- "Y/N likes me so there's no way that she's going to choose them over me. Right?"
- Seems like the type to just walk up to you and put his arm on your head and just hug you while staring at the person until the person just ends up leaving.
- "Hmph they weren't even that interesting"
- It's a petty move honestly, but it does work so there's that

Rengoku
-He has a fragile heart. Don't break it* (A/N you probably won't understand this if you haven't read the manga but read the end for the explanation!)
- "Hm! Y/N is talking to that person happily! I shouldn't interrupt them!" But in reality he feels jealous inside.
-He's the one to be a little annoyed later in the day and he doesn't even realize it
- "Rengoku you seem a bit down today?"

- "Hm? I do?"
- When you figure it out though, you make sure that you take extra good care of him that day.

Uzui
- god CONFIDENCE
- "That lowly commoner doesn't match with me!"
- But hey, even the most confident people have doubts sometimes
- I feel like he would be able to cope with it because I mean- he has three wives
- The thing is, I honestly think that he wouldn't think much of it, and you would never tell because he doesn't know it himself
- The jealousy is small but it's there

Genya
- Aggressive in a way
- "Hey she's taken fucker"
- Would 100% just directly shoo away the person if he felt jealous that you were getting hit on
- You scold him later for being too aggressive
- "She's taken" 
- It got better but it still isn't that subtle...

Iguro

- Ridiculing
- "Hey you really think that you can compare yourself to Y/N? Tell me a better joke than that"
- So much ridiculing, the person who was hitting on you would just uncomfortably- walk away
- It's kinda obvious
- No it's obvious
- Making fun of the other person so that he can feel superior. Probably
- But comfort him later

Gyomei

- He's pretty silent
- "I didn't like that person's aura"
- If you were ever hit on and he felt jealous, he'd probably just walk up behind you
- If his height and build didn't intimidate that person, then he would just- pick you up
- He probably would take you to a nice garden area and just relax there
- You end up just falling asleep but honestly it's enough comfort for him
- It shows that you're comfortable and that you can feel safe around him

Sanemi

- You know that one alignment chart where one of them is "Would stab as a warning"? Yeah that's him
- "GET FUCKED MOTHERF-"
- You probably had to hold him back
- If you couldn't hold him back then you probably had to talk him out of it
- He would punch that person in the face with no remorse
- It's kill or be killed I guess LMAO
- You comfort him later but he says he's fine
- tsundere

*Explanation for Rengoku having a fragile heart haha this is kind of my take on it but yeah (SPOILERS FOR THE KNY MANGA AHEAD)

During the train arc, there were kids who wanted to have "nice dreams" which just meant that the demon (Enmu I think) that they were working with would kill them off with no pain (They were kids with diseases or some kind of thing that was making their lives miserable).
So one of them were ordered to go into Rengoku's subconsciousness to stab his core (which would kill him in his sleep). The kid who went into his subconsciousness described Rengoku's core (I believe that it's also tied in with his spirit and heart) as weak and fragile. It was shown in the manga as well in that panel that his core was cracked. 
That's it for the explanation! Idk if this was any helpful but maybe just a fun fact haha