7(Demon Slayer)

An Au where Zenitsu and Kaigaku are blood siblings and were born and raised in the Red District until Kaigaku decided they should run away before Zenitsu reached the age to be bought for more then cleaning or playing an instrument(if Zenitsu is a woman or a just a very feminine looking with well done makeup and a kimono is up to the writer).

When they run away they run into both Gyomei and Jigoroku, who after hearing their story decided to take them in as their students(Gyomei took Kaigaku and Jigoroku took Zenitsu) but made sure the two still lived together seeing how protective Kaigaku was of his baby brother and how Zenitsu looked afraid of being separated from his brother.

The thing with Gyomei's orphans either didn't happen or it did with another character that would later take Kaigaku's place as demon.

While they both trained to be Demon Slayers, Kaigaku refused to take the test sooner and waited until Zenitsu was ready which their teachers advised them that it could be a bad idea but the two siblings said that if they had to die that they would do it together(just let them have a cute, wholesome sibling relationship) as neither would be here if it wasn't for the other.

Things follow canon but with the addition of Kaigaku in the Kamaboko Squad and that he and Zenitsu mostly fight together, until the Red District Arc.

When they heard were the mission was about, Kaigaku almost tried to kill Uzui for dragging them back to the place they run away but Zenitsu stopped them and reminded them that maybe they could save some children who were in the same situation and that there is no way anyone would recognize Zenitsu as he was known to have black hair and not blond(so yes, Kaigaku will dye his hair for this mission but what colour is up to the writer).

So they accept the mission but it is Kaigaku and Zenitsu who get them dressed to be bought since they know what the houses were looking for besides beating into Inosuke a few etiquette lessons and rules.

I would like to have Kaigaku and Zenitsu end up fighting Daki and Gyutaro(as here they would be more of a counterpart to them then Tanjiro and Nezuko), they don't need to be the winners or the demons' killers but I want ti to be shown how their backstories can be considered similar and how far they would go for their siblings.

Anything else in this Au would be up to the writer(also, if you wanna add Gyomei and Jigoroku giving their piece of mind to Uzui for taking their students to the Red District...I would enjoy to see it)