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.
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.
They might be confused at first. RWBY has its own unique lore and concepts that are different from many anime. So anime characters may need time to understand things like Grimm and Huntsmen. For example, if Goku from Dragon Ball was watching, he'd probably be like 'What are these strange monsters?'.
They could be surprised. The fanfiction might show them in scenarios they never imagined, like a shy character being in a very bold and adventurous situation.
Well, if we go by the name, the main character is Tobi for sure. And as for the other part of the love story, it could be any of the Akatsuki members. It could be Konan, who has her own charm and powers. Or it could be someone like Kisame, though that would be a really strange pairing. But in fan - made stories, the possibilities are endless when it comes to pairing Tobi with an Akatsuki member.
Definitely Kakashi. Since he has a connection to both Naruto and Tobi (Obito), his presence can add a lot of depth. He can be the one who tries to bridge the gap between Naruto and Tobi, or he could be a source of conflict as he has his own history with Tobi.
A few might be angry. If the fanfiction shows them in a negative light or changes their backstory in a way they don't like, they could get really mad. For example, if a character with a noble past is made into a villain in the fanfiction, they'd be furious.
The main characters are clearly Tobi and Sakura. Tobi is the one doing the tying up, which implies he has some sort of agency in the story. Sakura, on the other hand, is the victim of this action, and she will likely be the one trying to either resist or escape from this situation.
For Konan, her relationship with paper and her use of paper - based jutsu is a big part of her character. Writers should incorporate this into the story in a meaningful way. For example, if she is in a fight or a tense situation, her paper abilities should be used in a way that is consistent with how she uses them in the anime/manga. Tobi's use of his unique abilities like intangibility and teleportation should also be written in a way that makes sense for his character. His way of speaking, which is often a bit cryptic, should also be maintained in the fanfiction to keep him in - character.
Some Avengers might be really confused at first. For example, Tony Stark would probably be intrigued yet find it a bit odd. He'd be analyzing it like he does with new tech, trying to understand the appeal and the different universes within the anime fanfiction.