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During one of his missions, Tanjirou finds a little boy (definitely no more than two years old) who just lost his parents to a demon.
The scene is so heartbreaking and the story so familiar that Tanjirou's inner omega instantly feels a connection with the little one. The kid likes him, he immediately jumps into the security of Tanjirou's arms and doesn't let go of him after that.
Now, Tanjirou knows there's probably someone better suited to take care of the child; the omega doesn't even have a mate so he can't welcome the kid into a stable family, but his inner omega has decided that the kid is his pup now and that's the end of it.
He comes back to the butterfly estate and takes his pup to the girls to make sure he's completely fine. Tanjirou needs to be at his side at all times because the boy seems to get nervous around anyone who's not the omega.
Tanjirou learns that his name is Kiyoshi after the little kid grows comfortable enough to start speaking. It takes a while for him to trust Nezuko, but it helps that she smells a lot like Tanjirou.
When the little one starts calling him Mommy Tanjirou can't stop himself from tearing up a bit before nuzzling happily against the kid's cheek.
He's always wanted to have pups, but since he's not mated and no one seems to be interested in him, Tanjirou decided to push that thought away for the time being.
Sometimes he needs to leave his Kiyoshi in the butterfly estate, however, everyone has grown so fond of him Tanjirou knows he's safe there.
The Pillars are particularly nice to his pup, they bring him toys and play with him every time they see him. Tanjirou has found himself watching them interact with his pup with a fond expression on his face.
"Listen, I'm not trying to say they don't love Kiyoshi, we all do, but the hashira are really trying to impress you," Aoi blurts out one day, like she can't keep it inside anymore.
"What do you mean?"
"You must be kidding me, Tanjirou!"
The omega doesn't understand and it confuses him even more when he realizes she's quite irritated. However, he gets distracted because Kiyoshi happily runs towards him, giggling before hiding behind his legs because the Pillars are playing hide and seek with him.
"Mommy, don't say!"
"I won't tell them anything, little one," Tanjirou smiles before putting a finger against his lips.
It's obvious there's a very happy, very excited little kid giggling behind him, but Kanroji pretends to be disappointed when she doesn't find him.
"Mommy, he's the villain!"
For some reason, Shinazugawa always plays the villains and the monsters in their little games. He growls, although it's the softest growl Tanjirou has ever heard coming from the alpha.
He smiles at the wind hashira, trying to let him know how grateful he is the alpha is playing with his pup and he notices that Shinazugawa blushes and gets a little bit flustered for a moment before pretending to be defeated by Himejima.
"Let's go! Take Mommy too!"
Before Tanjirou can say anything, Rengoku takes him in his arms and Tomioka carries a very excited Kiyoshi to the safety of their "castle".
They all end up inside, taking a break while they have lunch before the pup falls asleep on Tanjirou's lap.
"You have done such a good job taking care of the pup, Tanjirou," Kocho comments looking a little bit nervous suddenly. The omega has never seen her like that. "So I don't mean to imply you're not, but have you ever considered choosing a mate to help you raise him?"
The other Pillars stop what they're doing in order to give him their undivided attention; it's like whatever Tanjirou is going to say is very important to them.
"Of course, I want him to have a Dad or perhaps a Mom," he grins for a moment before turning bright red and rubbing the back of his neck: "Besides... I want to have a big family..."
"I can be your alpha," Uzui immediately offers as Tokito tries to throw a rock at him before volunteering himself to be Tanjirou's alpha as well.
Everyone else does the same; they're pushing others and baring his teeth at each other, trying to tell him why each of them would be the best choice for him.
"You're really kind," Tanjirou says, smiling fondly at them. "I know you have grown fond of my Kiyoshi, but you don't have to make the sacrifice of mating me in order to keep taking care of my pup. You're already like family to him..."
"Sacrifice?" Iguro huffs, outraged. "Who told you such a ridiculous thing? You're anything but a burden, Tanjirou!"
"Do I need to knock some sense into you?" Shinazugawa almost growls, biting Tanjirou's cheek carefully. "Anyone would be fucking lucky to have you as their omega!"
They're really good friends.
"Thank you, but there's no need–"
"We want to!" Rengoku finally says, startling him. Fortunately, his pup is still asleep, although the butterfly girls come in to take him back to his room.
"What do you–"
"We're in love with you, darling. We'd love to have Kiyoshi as our pup too but the main reason we want to mate you is because we love you."
Slowly, the blush on Tanjirou's cheeks spreads down his neck as Kanroji talks to him.
"Oh."
"It doesn't matter if you want to have more pups or not, we just want to be with you," Tomioka assures him, taking both of his hands in his.
"I can't... I couldn't possibly choose!" He blurts out, mortified. He loves them all too and it'd break his heart to have to mate one of them...
"You don't have to," Himejima assures him. "You can marry us all."
With a quiet gasp, the omega realizes that they're serious about it. He gets a little bit flustered and hides his face behind his hands before nodding and blurting out a very shy: "I'd love to!"
No one is surprised when he tells them, not even Nezuko; Aoi mumbles something like "finally!" and his Kiyoshi is just happy to have more moms and dads who probably would do anything for him.