Lucy would be a bit scared at first. She's used to dealing with magical creatures within the Fairy Tail world, but Ghost Rider is something entirely different. However, her curiosity would soon get the better of her, and she'd start asking questions about his powers and origin. Happy would probably hide behind Natsu, peeking out nervously. As for Wendy, she might be in awe of Ghost Rider's power, but also concerned about the potential danger he poses to her friends.
Ron would likely be more on the scared side at first. He'd be hesitant to get too close to the Ghost Rider, but as he saw the Ghost Rider using his power to fight against dark forces, he might start to respect him. Ron would probably crack some jokes along the way to ease the tension, like comparing the Ghost Rider's flaming head to a really over - the - top Weasley's Wizard Wheezes product.
Issei would probably be both scared and excited. He's used to dealing with powerful beings, but the Ghost Rider is on a whole different level. He might initially try to fight him out of bravado, but soon realize he's outmatched. Rias would be more cautious. She would see the Ghost Rider as a potential threat to her peerage and would try to figure out his motives before taking any action.
Hero characters might analyze it from a moral perspective. They could either approve or disapprove of the way the story is told in the fanfiction depending on whether it aligns with their sense of right and wrong. Villain characters, on the other hand, might look for ways to use the ideas in the fanfiction to their advantage, like new ways to create chaos.
A villain like Shigaraki would likely be interested in the power levels in the MCU fanfiction. He's always looking for ways to increase his own power, and seeing the various powerful artifacts and abilities in the MCU might give him some ideas. He could think about how he could use similar things in his own plans to take over the MHA world.
The Death Eaters would likely be both scared and intrigued. They might see the Ghost Rider as a potential new weapon or a threat to their own dark plans. Voldemort himself would probably view the Ghost Rider as a rival in terms of power. He might try to either destroy the Ghost Rider or subjugate him. The other wizards in the Ministry of Magic would probably panic and try to contain the situation without really understanding what they are dealing with.
Some MHA characters might be flattered. For example, Midoriya would probably be really excited and take it as a sign of how much he's inspired people. He might even look for positive lessons in the fanfiction to improve himself.
If the MHA characters were to react to a Mastermind fanfiction, some would be more accepting than others. Characters like Ochaco might find it really exciting. She's a fan of the heroes herself, so seeing different takes in fanfiction would be like getting a new perspective on the MHA world. However, characters like Shoto, who are more reserved, might be a bit hesitant. He might need some time to process what he's reading. But overall, it would be a very interesting scenario as it would show how the characters view different interpretations of their world.
Some characters would be shocked. For example, if the fanfiction had a really dark and unexpected Mastermind plot, like a hero turning out to be the mastermind of all the villains' plans, characters like Tsuyu would be very taken aback. She'd probably question the motives behind such a plot. Then there are characters like Denki. He might be more into it if it had some cool action scenes in the fanfiction, regardless of how out - of - the - blue the Mastermind storyline was. And Nejire would probably be super curious about the emotional depth of the characters in the fanfiction.
The story of Ghost Rider involves a person who becomes possessed by a supernatural spirit and gains supernatural powers to seek vengeance against evil. It's a tale of redemption and justice in a dark and supernatural world.