His friends from the village, like Shikamaru, Choji, and Ino, might visit Naruto. They would bring some small gifts or just come to chat with him to pass the time and keep his spirits high. Also, the Hokage might visit to show support for the hero who has done so much for the village.
Iruka - sensei might also come to see Naruto. Iruka has always been like a father figure to Naruto, and he would bring some comfort and words of encouragement. And Kakashi - sensei could also pay a visit. He would probably give Naruto some advice on how to recover faster and get back to his ninja duties.
His teammates, Sasuke and Sakura, would surely visit him. They are his closest comrades, and they would be worried about his condition.
Ino would most likely be there too. Since they have been friends since childhood, Ino would be worried about Sakura's condition. She might bring some flowers and stay by Sakura's bedside, chatting with her about their memories and the village.
His best friend Grover would surely visit him. Grover would bring some of Percy's favorite foods from Camp Half - Blood, like blue cookies. Annabeth would also be there. She'd probably be reading books on mythology, trying to find a way to help Percy get better faster.
Harry Potter could visit Snape in the hospital. Despite their past differences, Harry might feel a sense of obligation or curiosity. He could be there to finally have an honest conversation with Snape and perhaps gain a better understanding of his actions throughout the years.
Plagg would also be there. Plagg is Chat Noir's kwami, and he has a special bond with Adrien. He would probably be worried about Adrien's condition and try to cheer him up in his own unique, mischievous way, like making cheesy jokes or trying to steal some cheese from the hospital cafeteria.
His friends like Misty and Brock would probably visit him.
Hermione would surely visit. She'd be there with a stack of books, ready to help Harry catch up on his studies while he recovers. And Ron would be right beside her, probably with some treats from the Hogwarts kitchen.
Obviously, Naruto could be one of the main characters. He could be either a patient or someone helping out at the hospital. Another main character could be Sakura. Since she has medical skills, she could be a doctor or a medic - ninja in charge at the hospital. And then there could be a new character, like a young orphan who is being trained as a medic - ninja at the hospital and has an interesting backstory.
Another character could be Kakashi. He might be the one who doubts the decision to send Naruto to the mental hospital. He could secretly visit Naruto and try to train him in mental fortitude techniques so that he can get out of the hospital sooner.
Tsunade for sure. As the Hokage and also a medical expert, she would be furious about Naruto escaping the hospital. She might send some ninjas to bring him back.
I'm not entirely sure specifically as there could be many different interpretations of a 'naruto hospital lemon fanfic'. It might involve a story set in the Naruto universe within a hospital setting, perhaps with some original or unique plotlines created by the fanfic author.