In a fanfiction where Deku is hit with a truth quirk, there are many possibilities. Firstly, his interactions with his mentors like All Might would become very different. He might admit his doubts about being a worthy successor, which could either strengthen or strain their relationship. Secondly, his classmates would be shocked by some of the things he reveals. For instance, if he tells them how much pressure he feels to be the best. This could lead to more understanding and support among them, or it could create some jealousy and rivalry depending on how the other characters react. Lastly, Deku's own view of himself would change as he has to face his unfiltered thoughts and feelings.
I'm sorry, I don't know who the author of this fanfic is. There are so many fanfic writers out there, and without more information, it's impossible to determine.
Since I don't have access to the 'castiel hit in the nuts fanfic', it's hard to say exactly. However, it could be that the author wanted to add a bit of shock value or a very specific kind of physical humor to the story. In some fanfics, such extreme or unexpected events are used to break the routine and explore different aspects of the character. It could also be related to a particular sub - plot or relationship within the fan - created world where this incident has consequences for Castiel's interactions with other characters.
Deku's character development in this scenario is complex. At first, he may be embarrassed and scared by the things he can't help but say. But as time goes on, he learns to embrace this new state. He realizes that being truthful all the time, even about his weaknesses, can be a strength. His relationships evolve as his friends come to appreciate his honesty. He also grows in terms of his hero identity. Instead of trying to project a perfect image, he shows his true, flawed self, which makes him a more believable and inspiring hero in the fanfiction world.
He might cry a lot. Deku is very emotional, and being cheated on would be a very painful experience for him. He could also get angry, which is rare for him, but this situation would be so unjust that he might not be able to hold it in. He'd probably start to question his self - worth too, thinking that there was something wrong with him that made his partner cheat.
In fanfiction, if Deku gets kidnapped, his friends from U.A. High would likely launch a rescue mission. Bakugo might be especially determined, as much as he tries to hide his concern. They would use their quirks in creative ways to track Deku down.
There's also 'Injured and Alone'. In this fanfiction, Deku gets hurt while on a solo mission. It delves deep into his thoughts as he tries to survive in a dangerous situation, all while being severely injured. It's a great read for those who like character - centric stories when the character is in a tough spot.