Kakashi might be initially surprised. But then, he would probably use it as a teaching moment for Naruto. He could say something like 'Naruto, stealing is not the way of a ninja.' And then give him a punishment like extra training.
The reactions of pro heroes to Deku fanfiction could vary greatly. Pro heroes who are more outgoing and friendly, like Mirio, might actively seek out Deku fanfiction just for fun. He could be the type to laugh out loud at the creative and sometimes wacky ideas in the fanfiction. However, more serious pro heroes, such as Aizawa, might initially be skeptical about the whole concept of fanfiction. But if the fanfiction has some deep insights into Deku's character and his journey, Aizawa might start to respect it more.
Kakashi's reaction would be complex. He is a man who has lost many people in his life. But Naruto was special. He would hold Naruto tightly, his hands trembling slightly. He might try to use some of his own chakra to keep Naruto alive a little longer, but deep down he knows it's futile. His face would show a mixture of pain, regret, and love. He would remember all the times he trained Naruto, all the lessons he taught him, and how Naruto had surpassed all expectations.
Kakashi would likely be very concerned. He would immediately assess Naruto's injuries. If they were serious, he would use his medical skills to start treating him right away. His first reaction would be to make sure Naruto was stable and out of immediate danger.
Kakashi might immediately offer a comforting presence. He could put his hand on Naruto's shoulder and just be there silently, letting Naruto know he's not alone.
Kakashi may also get angry. Angry at the people or things that caused Naruto to get those scars. This anger could fuel his determination to make Naruto stronger, so that he can better defend himself in the future. He might then intensify Naruto's training, all while having a new - found sense of protectiveness towards Naruto.
At first, Kakashi might try to distance himself from Team 7's possessiveness. He values his independence and doesn't want to feel smothered. However, as he realizes that their actions are coming from a place of deep love and loyalty, he starts to accept it more. He may even start to rely on them a little bit, for example, allowing them to help him with his paperwork or other small tasks that he usually does alone. This shows a new level of trust and closeness between them.
The theme of the underdog and the idol is also common. Izuku is the underdog, and the female pro hero is someone he looks up to. Their relationship then blurs the line between admiration and love. For example, if it's with Mirko, Izuku admires her brash and fearless fighting style. As they interact more, their relationship can change from that of a fan - idol dynamic to a romantic one. There can also be a theme of sacrifice, where either Izuku or the female pro hero is willing to sacrifice for the other in a dangerous situation.
Well, Kakashi is a complex character. When he discovers Naruto is abused in the fanfiction, he may first feel a deep sense of guilt for not noticing earlier. He'd then use his skills and influence in the village. He could start by reporting it to the proper authorities in Konoha, like the Hokage. And at the same time, he'd be there for Naruto, providing emotional support, talking to him, and helping him through the trauma. Kakashi has his own past traumas, so he'd understand what Naruto is going through on some level.