Well, there are also those characters who might be a bit shocked or confused. Take Levi from Attack on Titan. He's a very serious and no - nonsense type of character. If he were to read some of the fanfic about him that deviated too much from his own self - perception, like being made into a comical or overly emotional character in the fanfic, he might be taken aback at first. But then he might also realize it's just a different interpretation and not take it too seriously.
Well, it depends on the nature of the fanfiction. Some anime characters might be amused if the fanfiction is light - hearted and full of comical situations. For example, if it's a fanfic about a cooking competition among the characters from a shonen anime, they might react with good - natured laughter and competitiveness. They could also be confused if the fanfiction takes their story in a very strange or unexpected direction, like suddenly making a character from a historical anime end up in a space adventure.
They might be shocked. Luffy has so many wild and dangerous adventures in his memories. His friends would probably be in awe of how much he has endured and how strong his will is.
The characters' reactions can be quite diverse. Chopper would be extremely worried and immediately start his medical examination. He'd be constantly checking Luffy's symptoms and trying different remedies. Nami would be worried too but also thinking about how this could affect their journey and the crew's safety. Zoro might seem nonchalant on the outside but would be secretly on high alert, ready to fight off any danger. Sanji would cook up all kinds of nutritious food to help Luffy get better.
Usopp might be inspired to be braver. When he watches Luffy's memories, he could see how Luffy fearlessly faces powerful enemies. This would make Usopp want to overcome his own cowardice and be more like Luffy. And Robin, she would gain more insights into Luffy's kind heart. She might see how Luffy has always been inclusive and accepting of everyone, which would make her feel even more at home in the crew.
Zoro would be deeply affected. He has sworn to follow Luffy and his death would be a huge blow. He might feel like he failed in his duty to protect the captain. He would likely become more withdrawn at first, but then focus on getting stronger to avenge Luffy if there was someone to blame for his death.
There are some doujin novels about Luffy that I can recommend. For example, One Piece Liar, Starting with the Devil's Fruit, was a Doujinshi novel set in the real world of pirates, mainly centered around Baki, Smoggy, and the others. In addition, One Piece: I'm Luffy's Twin Sister was a story in which the male protagonist was reborn as Luffy's twin sister. In addition, Doujin novels such as One Piece: Luffy Godfather and One Piece: I'm Luffy Wang could also be considered for reading. However, the quality and content of these novels were up to the readers to judge.
Kakashi would be very observant. He would look for how the fanfiction writers understand the world of Naruto, the values and the moral lessons. He might even use some of the creative ideas from the fanfiction in his own teachings if he thought they were valuable and in line with the spirit of the original story.
There could be characters who are outraged. If the fanfiction distorts their personality or makes them do things completely against their nature, they may be very angry. For example, a noble character made to act like a coward in the fanfiction would likely not take it well.