The Hashiras would likely show concern. Some, like Shinobu, would immediately start looking into possible medical solutions. Giyuu might be more reserved in his worry but would still keep a close eye on Tanjiro. The other demon slayers would also be affected. They look up to Tanjiro, so his sickness would lower the morale of the group. They would try to support each other while waiting for Tanjiro to recover.
Dan would probably be shocked at first. He has a complex relationship with both Lucifer and Chloe. But he would also be concerned for Chloe's well - being and the future of the baby. He might offer his help in more practical ways, like making sure Chloe is safe during her detective work. Ella would be super excited. She's always positive and would probably start knitting baby clothes right away and be full of ideas for baby names.
Their friends would probably be really happy. Amenadiel might be a bit surprised at first but then very supportive, seeing how much Lucifer has changed because of Chloe. Dan would be a bit more hesitant, but eventually come around as he sees how happy they are. Maze might be a bit jealous at first, but then she would also be happy for them as she cares about Lucifer.
Tanjiro might use his sense of smell to detect hidden threats. Since his sense of smell is super - enhanced, he could sniff out any demons or traps in the way of saving Rengoku. For example, if there are demons disguised as allies, Tanjiro would be able to tell by their scent.
Dumbledore would be very cautious. He would see Harry's reincarnation as a great risk but also an opportunity. He might try to guide Harry, but also keep a close eye on him to prevent any disasters. The Malfoys would likely see this as a chance to bring down Harry and his friends. They would spread rumors and try to turn the wizarding world against him. Harry's other friends from Gryffindor would be a mix of shocked, scared, and some would be brave enough to stand by him, believing that Harry is still the same person deep down.
Yes, in some fanfictions, Tanjiro is depicted as the reincarnation of Yoriichi. This concept adds an interesting layer to the story as Yoriichi was an extremely powerful and legendary figure in the original lore. The idea allows for new storylines where Tanjiro might inherit some of Yoriichi's skills or have a deeper connection to the past events related to Yoriichi.