Basically the demons and the humans swap places
Breathing style + weapon: ice breathing, he uses bladed fans
Appearance: fair skin 6’1 long platinum blonde hair, standard demon slayer uniform he wear a red haori with a darker red in a blood dripping pattern (like his shirt in demon form) rainbow eyes
Personality: he puts on a cheerful and care free persona however underneath that he’s cold, and struggles to show his real emotions, he’s not a psychopath in this AU he holds a lot of hatred for Shinobu and determined to kill her.
Backstory: Douma was born into the eternal paradise faith cult and was thought to be a messiah where he would listen to people troubles and give advice, however his advice would sometimes be wrong and this angered people and lead them to believe that he was a false prophet, so when Douma was 8 they attempted to sacrifice him. He was saved by Kotoha Hashibira a 15 year old who was the wind Hashira at the time and she took him in as her tsuguko. Kotoha was later killed whilst on a mission to kill two upper moon she kill upper moon 10 (Kanae) but fail to defeat Upper moon 9 (Shinobu). A ten year Douma was left to take care of baby Inosuke. Several years later when Douma was 20 he took in Gyutaro and Ume raising them and training them.
Additional info: runs a medical facility out of the Ice estate, took Kotoha’s last name 2nd strongest Hashira ice breathing lowers your body temp. so if Douma uses it for to long without breaks in between he will die. Some of the other Hashira find him annoying for his carefree attitude about everything, 2nd oldest of the Hashira