Tanjiro:
How he got hurt: Inosuke broke his ribs.
The two got into what Tanjiro figured was a friendly spar. Inosuke was pissed about something your boyfriend had done, and so when he let his guard down, Inosuke charged right at him and rammed his head into Tanjiro's chest, cracking two of his ribs.
You had to laugh when Zenitsu started shouting at him for it, chasing him off so you could check on Tanjiro. He was a pretty good sport about the injury, letting you check him over and help him.
Really, though, what's with these Slayers, breaking ribs and getting concussed like it's your average Tuesday?! It was almost enough to get you to retire early just out of sheer exhaustion!
Nezuko:
How she got hurt: Tanjiro's forehead.
Tanjiro overslept, so Nezuko went to wake him up. She shook him before trying to headbutt him in anger. Unfortunately, his head was hard enough that he, first off, didn't wake up, second, caused her to start bleeding.
She went to you, eyes tearing up and glaring at the ground. You tried your best to soothe her, instead getting Zenitsu to wake him up with his shouting. You bandaged her forehead and Tanjiro apologized to her, looking quite guilty for it. She simply rubbed her face in his chest as if to say "I forgive you".
Zenitsu:
How he got hurt: he accidentally cut himself.
The two of you were doing a speed-chopping contest with the carrots you got as a reward for protecting a poor farmer and his daughters from a demon. Zenitsu was winning (key word was), before his hand slipped and the knife went through his index finger.
He immediately shouted in pain and dropped the knife, beginning to cry from the pain. You quit chopping the carrot to look him over. The sheer amount of blood was a little concerning, but luckily all he needed was some stitches. He'd done a number on his finger; you had to be a little impressed.
After it was said and done, you kissed the bandage to make it better and abandoned the chopping contest, instead taking your chopped carrot pieces for the group to eat as a snack.
Inosuke:
How he got hurt: he choked on his food.
To celebrate taking down a Lower Kizuki, you cooked the gang a special meal (with Nezuko's help guarding the kitchen from any would-be food snatchers). Once you all started eating, Inosuke blew through his portion with vigor, causing some concern between you and Zenitsu.
He seemed okay for the most part until he suddenly dropped his chopsticks on his plate, gasping for air. and weakly coughing. You realized pretty quickly what was going on and leapt from your chair, grabbing him around the waist and hauling him away from the table to do an emergency Heimlich. Thankfully, he spat out the hunk of meat and slumped over in your arms, exhausted. You made sure he was okay before letting him go back to the table.
Giyuu:
How he got hurt: he pissed off Sanemi.
The poor bastard just wanted to make up with the wind pillar over some slight or another. He tried giving the man some homemade ohagi, but he got his ass kicked anyway. He limped back to you, feeling a little worse than before and with only broken fingers and a squished ohagi. To be blunt, you were pissed. It took you hours of trying to make it perfectly, and he just squishes it and kicks your boyfriend's ass?
You patched up Giyuu, gave him lots of soothing hugs (and a kiss on the cheek, at his shy and awkward non-verbal request), and then proceeded to march up to Sanemi and verbally beat him down three ways to Sunday.
Kyojuro:
How he got hurt: he accidentally burned himself.
How ironic. A flame pillar, the best of the best in his field (and the only practicing user, but tomato tomato), one of the strongest slayers alive... and he's taken down by a campfire.
Kyojuro and you were on a little romantic getaway in one of your family's mountain cabins when you both decided to start a campfire and roast some meat and bake some apples. You'd done it many times before, so Kyojuro was the one in charge of finding some prime roasting sticks from the surrounding area. All was going well... until Kyojuro accidentally pushed his feet too far near it and gave himself second-degree burns.
He was pretty good-humored about the whole affair, even after you panicked and grabbed him and tossed him into the lake nearby to soothe the burn as a reflex. He simply laughed off your apologies, letting you help him to the house so that you could treat his burns and wrap his feet.
Muichiro:
How he got hurt: he got decked by a bear.
You read that right: Muichiro got pimp-slapped by an irritated bear. How, you might ask? The boy himself doesn't even know. The two of you were just walking along and you came across a bear lying in the sunlight. Muichiro wasn't paying attention, so he accidentally stepped on the bear's paw and woke it right up. In retaliation, the animal roared and swiped at him with a paw, hitting him clean in the face and knocking him back. By the time you helped him back up, the bear was gone.
The two of you were stunned by the sudden move, but it soon devolved to you both laughing so hard you fell to the grass again. Luckily, he only had light lacerations across his face and would heal up just fine. The incident became an inside joke prime for confusing the other pillars.
Shinobu:
How she got hurt: she got the stomach flu.
Generally, Shinobu has made herself pretty much immune to regular ailments like colds and being poisoned, but one day you visited her only to be warned by Aoi that she was sick. You rushed right in to see her sitting in her room, pale and sweaty, with an untouched cup of tea before her.
She explained tiredly that she'd been sick all morning, and she could keep nothing down. You offered to nurse her, but she laughed gently and declined. She said you could stay around if you wanted, so you did. You kept her company during the times when she wasn't rushing to the bathroom.
Gyomei:
How he got hurt: he accidentally headbutted you.
The two of you were in battle and you were too busy fending off the demon in front of you to realize where Gyomei was. You turned to see how he was at the same moment when he leapt to your aid, causing the two of you to knock noggins and fall back.
He luckily recovered faster than you, defeating your foe before you struggled to your feet. He apologized for hitting you and lead you back home.
Sanemi:
How he got hurt: he got food poisoning.
Karmic retribution for insulting your cooking, methinks? Sanemi said your egg rice sucked one day, and the next brought him right to your door, looking horribly ill and asking for your assistance. Apparently, he ate some bad sushi and had been curled over the toilet all day before Shinobu told him he might as well just go to you so that you could monitor him. He was too weak to snark back and just did as told.
After your five-second feeling of 'what are the odds' and chuckling at it, you felt bad for him and let him hole up in your guest room all day with a sick basket, bringing him water and easy foods and just comforting him.
Akaza:
How he got hurt: he got stabbed in the throat.
You were out and about at night when a demon snuck up on you two with the intent to kill and eat you. Akaza defended you, but the demon wasn't amused and stabbed him in the throat with his... bone... knives. (Think Kimimaro's bone technique, but oni). It was very gross, but very, very cool.
You managed to kill the demon and tried your best not to panic, going over to him to see if he was alright. He healed it the best he could, but he was still a little grumpy for the rest of the night. (You eventually had to get him a small jar of blood to make him feel better... it wasn't from you, it was a favor called in by Shinobu. Whose blood it was, you didn't want to know.)
Douma:
How he got hurt: you accidentally stabbed him.
To your credit, it was absolutely an accident. You were training with your katana and he snuck up on you, causing you to whirl around and stab him in the shoulder. When you realized it was him, you freaked out, apologizing as he stood there, momentarily stunned.
Fortunately, he wasn't that mad about it. Actually, he seemed to like? it?? You stood there in surprise as he started talking about joining you during your training (with a weird blush on his face... you weren't sure if you should be worried or relieved).