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.
The Quinx Squad could react with a mix of respect and wariness. They respect power, and Kaneki has a lot of it. However, they are also loyal to their cause of protecting humans from ghouls. Meeting Kaneki would force them to re - evaluate their views. They might start to question whether all ghouls are truly bad, especially when they see that Kaneki has his own moral code. This could create internal conflict within the squad as they try to figure out how to deal with Kaneki.
Harley Quinn might get really into it. She has a wild and unpredictable personality. She could start to role - play some of the scenarios from the fanfiction in real life, much to the confusion or amusement of the other Squad members. It would be in - character for her to embrace this strange new view of their world.
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.
Toge Inumaki is important too. His special cursed speech ability makes him stand out. Also, the principal of the school, Masamichi Yaga, is likely to be important as he is in a position of authority and has an influence on Gojo and the other students. There could be other characters like the members of different clans that also play roles in the complex web of the story.