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.
Their friends would likely be very concerned. Shikamaru, being Temari's close friend and Naruto's teammate, would try to use his intelligence to figure out what's wrong. He might start by just spending time with them, not pushing them to talk right away. Sakura, as Naruto's long - time friend, would probably be more direct. She'd try to get Naruto to open up about his feelings, using her medical knowledge to also check if there's any physical ailment contributing to his depression. For Temari, Ino could reach out, as she's good at getting into people's heads. She'd offer Temari support and try to make her feel better about herself.
His friends might initially be shocked. For example, Sakura might feel guilty for not noticing sooner. She could try to be more present in Naruto's life, cooking for him or just sitting and talking. Shikamaru, being the perceptive one, could come up with a plan to cheer Naruto up, like organizing a small get - together with just their closest friends.
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.
Ino, being her former rival and now friend, could be very empathetic. She might try to get Sakura out of her shell by taking her shopping or just having girl - to - girl talks. Kakashi, as their teacher, would probably be observant. He could give her special training or tasks to boost her confidence. And the other members of their ninja team would likely offer their support in different ways, like sharing their own experiences or just being there for her.
Harry often tries to be there for her. He might not fully understand her depression at first, but he'll stay by her side, offer a listening ear and try to cheer her up in his own clumsy way, like suggesting they go on an adventure or something.