Tanjiro:

Depending on the injury, he's gonna ignore it. The boy's had worse from far worse, a simple cut or sprain isn't going to stop him when he's focused.

If he has to drop out of the fight, expect him to be stubborn about it. He wants to help you; it's gonna take a lot of convincing for him to stay back until he can get fixed up. Nezuko and the others are gonna be a lot of help if Tanjiro goes down. It takes a little longer to finish, but as soon as the demon dies you haul him off.

When he wakes up from his injuries, he'll be so apologetic about the whole thing, as if he accidentally tripped you instead of getting so hurt that he has to visit the Estate. No matter how much you scold him, he keeps that gentle smile on his face, still able to smell your worry for him. He's very touched by your concern.

Nezuko:

Depending on the injury, she might not react much. Of course if Tanjiro gets swiped at, she's gonna get angrier at the demon in question. She can heal quicker than you or Tanjiro could, and to a point, she doesn't care about throwing herself in the way of danger -- if it's to save her friends, she'll do it, no matter how much you beg her not to.

If the injury is bad enough that she has to drop out of the fight, she won't stay back for long if she can help it. The only way to keep her from trying to join the fray again is to knock her out or hope she falls unconscious, because either way, she's probably planning to try something as a last-ditch effort, like using her Blood Art. Either way, after the fight is over, Tanjiro gets into contact with Lady Tamayo to see if she can help. If she recommends otherwise, then it's off to the Estate with your group.

Once she heals enough, expect her to be more affectionate than usual towards you: hugging you, leaning on you, maybe fussing with your hair. Nezuko feels awful that she made you so worried about her and will try to apologize by being with you. She can't promise that she'll practice better self-preservation ("Really, first Tanjiro, then you," you'd muttered in annoyance, "what's with you Kamados, anyway?"), but she tries not to worry you so much.

Zenitsu:

Either way, Zenitsu is going to panic a little. He'll most likely scream at first, catching you off-guard -- but he quickly gets back into battle. If it's bad enough that he needs to stop, he'll grow more serious as he watches you all fight. As soon as it's over, he'll be more than happy to soak up your presence while you carry him off to the Estate.

Once he wakes up in the morning, he cries a little, apologizing profusely for crippling you like that. Of course, he's happy if you express concern over him regardless, but it doesn't stop the guilt of having to drop out like that. He wishes he could do better to impress you.

Inosuke:

Depending on the injury, he'll shrug it off -- he's lived in the mountains all by himself for years, he's had worse before. It would only serve to fuel his energy towards your opponent. If it's bad enough that he needs to drop out, expect him to be shouting encouragements (in his own brash way, of course) to the rest of you. Once the fight is over, you'll have to haul him off to the Estate. He'll most likely be stubborn about traveling, so depending on his injury, expect to at least be his crutch.

Once he wakes up in the morning, he'll be a little sulky about having to sit and watch as the rest of you defeated the demon. You'll have to give him lots of encouragements and praise to get him back to his usual self. Praise is a must with Inosuke, especially if you call him 'Inosuke-sama'.

Giyuu:

Depending on the injury, he won't react much to it. He's had worse before; what's a simple scratch or bruise? But if it's bad enough that he needs to stop, he'll be very reluctant, but in the end he'll respect your pleas and watch from a distance. He may call for backup, depending on how tough your opponent is; he doesn't want to watch you fight someone like a Lower Moon alone. You aren't even a pillar, nor do you have any official training. You were just caught in a fight with nothing else but your budding (breath style) skills and your father's blade.

In the end, once the fight is done, you may have to be his crutch as you either head back to your parents' or to the Estate. Either way he's going to have to do some resting up, whether he wants to or not. He's surprisingly compliant with you, being too tired to argue his own well-being.

Once he's rested, he'll listen to your complaints about his overzealous behavior with a twinge of guilt. He doesn't want to trouble you any more than he already has to, yet he feels like he deserves the verbal lashing. Once you're done giving him a piece of your mind, you hug him tight, and it flusters him for a few moments before he tentatively hugs back.

Kyojuro:

Let's be honest: Kyojuro would try to keep going, even if he were really hurt. If your opponent was a little too challenging, he may consider a call for backup or even a tactical retreat, if it really called for it, but he's not going down without swinging.

If the injury was bad enough that he had to stop fighting, depending on the opponent he may be uneasy about you fighting alone. While you're incredibly skilled in (breath style), he knows that you have your limits. Either way, once the fight is done, he'll be extremely proud of you for managing and will accept your bid at being his crutch back to the Estate.

Once he's rested enough, he'll tell you about how proud he was of your feat. What's something like this to keep him down for long? He listens to your concerned rant with a soft smile, and when you're finished, he pats your head. You remind him much of his little brother, and it makes him want to shield you even more so that you can hone your skills with his protection.

Muichiro:

Muichiro is going to keep fighting, no matter the injury... unless it knocks him out first. He's trained too hard and spent too much time honing his skills to run away from a fight. It's almost concerning about how little he cares for himself compared to his allies. Of course, if civilians are in the crossfire, he'll do what he can to direct the demon's attention elsewhere so that they can escape.

He doesn't want you to have to fight alone. Tanjiro did well to 'knock some sense into him', so to speak. You're his friend now, and he would never leave you to try and take down a demon by yourself. He recognizes your capability as a slayer, of course, but you're not a pillar yet -- technically speaking, he's still your superior, even if he is younger.

Once the fight is over (and well-fought), you'll probably have to act as his crutch to the Estate. Once he's rested, he may compliment your skills and thank you for bringing him to safety. His focus is no longer just on himself, but on others as well, and it warms your heart to know that he's a dependable ally now.

Shinobu:

Shinobu can more than handle herself; she won't give injuries much thought unless they're poisoned in some way, and even then, she handles it quite well. If her wounds are bad enough that she needs to drop out of the fight, she does so with grace, possibly calling for backup so that she can get some medical help.

Once the fight is over, the two of you go back to the Estate, where she can rest up and where you can spend the night. When she's rested enough, she may good-naturedly poke at you for being so worried about her, but she does take your concerns seriously. She doesn't want to lose you either.

Gyomei:

He's not going to go down without a fight, although he does acknowledge when he needs to back down. He's the strongest pillar by far, though. It's gonna take a lot to make him stay down.

When he does need to back out of the fight, he prays for your safety, watching as you work to kill the demon. When you do manage to kill it, he praises you, thanking the gods for your safety. The two of you either go to your parents' home or to the Estate. He doesn't mind if you stay with him while he rests up.

When he is rested, the two of you have a talk about your combined skills. He recognizes you as an up-and-coming fighter with good skills, although he feels rather uncomfortable leading you headlong into danger, moreso because he knows your parents quite well by now. Still, he accepts that you want to fight beside him and thanks you for being so committed.

Sanemi:

Sanemi is stubborn as hell. He's not going down without swinging with all he's got. Sometimes you have to try and knock him out to get him to stay out of it... key word 'try'. He always pushes his limits when it comes to his opponents, preferring to taunt them and pick at their defenses until they give him what he calls a 'real fight'. Despite Nezuko proving herself to be trustworthy, he still hates demons with all his might and he will not go easy on them.

Still, in the very, very slim chance that he has to watch you fight from the sidelines, he'll be hollering a lot of encouragements, peppered with coarse language here and there. He worries a lot about you, and fighting a demon alone is nothing to sneeze at. He doesn't want you to get badly hurt, but he'll disguise it with heckling. Still, once the fight is over, he grumpily acknowledges that you've got skill and lets you drag him to the Estate to heal.

Once he's rested, you may get into an argument with him about backing down versus keeping on fighting when you're at your limit. He gets angrier than you, demanding that you mind your own business about his tactics and to let him do what he needs to do. More often than not, you have to leave and come back later to cool off, and when you do, he begrudgingly apologizes for what he said and accepts your worried feelings.

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A/N: I can't believe this has 100 reads already. You're some real ones!