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.
Well, in the fanfic, Bruce would probably become more withdrawn. He'd brood over Jason's death, questioning his methods and his role as a mentor. Dick would be angry at the world, especially at the villains who might have been involved. Tim, on the other hand, would be more determined to prove that he can be a worthy Robin, but also carry the pain of losing Jason. Alfred would be the one constant, trying to comfort everyone with his wisdom and care. It's a really tragic event for the Batfamily and their reactions would be complex and full of emotions.
The guild would be devastated. Natsu would probably be one of the first to be enraged. He'd vow to find whoever or whatever was responsible and make them pay. His fire would burn hotter than ever as he channels his anger.
Gray would be devastated. He might go into a state of deep depression, isolating himself from the rest of the Fairy Tail guild. He would blame himself for not being able to protect her, especially if her death was related to her love for him. Gray would probably be filled with anger as well, and he would vow to take revenge on whoever was responsible for her death.
Mr. Schuester would be in shock. He saw Rachel as one of his star pupils and a driving force in the Glee club. He would struggle with the guilt of not being able to protect her. He would try to keep the club going as a way to honor her memory, but it would be very difficult for him. He might also start a scholarship fund in Rachel's name for future Glee club members.
Well, through the Dance of the Fire God, Tanjiro's character develops in multiple ways. Firstly, his sense of purpose becomes even more defined. He realizes the importance of the dance not just for himself but for the whole world he's trying to protect from demons. His compassion grows as well. When using the dance, he might see the pain of the demons more clearly, which in turn makes him more empathetic. His leadership skills start to emerge too. Other demon slayers might look up to him when he showcases the power of the dance, and he has to take on more responsibilities. Also, his determination gets a huge boost. Each time he performs the dance, he is more determined to end the reign of demons.
Well, it can be considered a spoiler if you haven't reached that part of the story yet. But if you're okay with knowing some key details upfront, then maybe not.
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.