Demon King/Queen Tanjiro and Kanao
7 parts Ongoing Tanjiro and Kanao were turned into demons after the final battle against Muzan. Like themselves everyone was confused on how did Kanao and Tanjiro turned into demons. The reason Tanjiro and Kanao are demons is because Muzan injected half of his blood into both of them but he thought that he would be in control of them but no they were in control of themselves. The others thought they needed to kill them but their friends and the Hashira knew they weren't bad so went with Kanao and Tanjiro. Tanjiro and Kanao eventually got married same with; Inosuke & Aoi, Tomioka & Shinobu, Zenitsu & Nezuko and Iguro & Mitsuri. Mitsuri and Iguro Started to trust Kanao and Tanjiro after 4 months. Shinobu, Kanae, Giyu, Genya, Mitsuri, Igure, Sanemi, Nezuko, Zenitsu, Inosuke and Aoi became demons.
(This story originally belong to Nanashimura1)